enigmatic puzzle - Piece de Resistance - Six-six-six. An Ultimate Wall of Cryptics

Six - six-six. An Ultimate Wall of Cryptics

_{This puzzle is part of the "Piece de Resistance" series. Go back to Part 1 (Ace) for the story.
Ace Two Three Four Five Six ...}

Aha! This one is of landscape orientation! Let's see...

Hint 1:

Title is hint.

Hint 2:

Threaten chickens to be devils

Answer

Suggested answers for all 16 cryptic clues follow below. Note that several of the clues employ 'printer's devilry' techniques, as suggested by '666', the mark of the beast from Revelation, appearing in the title.

Originally upset, Peter is awake (2)

UP (‘originally’ as in ‘first letters of’ Upset, Peter, meaning awake)

Ultimate veto in switch position (2)

ON (‘ultimately’ as in ‘last letters of’ vetO iN, meaning switch position)

They opposed stating threats (3)

EVA (printers devilry: They oppose dEVAstating threats) - solved by @JS1 in comments

Intend to wrap up alibi and lead mystery (3)

AIM (‘wrap up’ as in ‘ first and last letters of’ AlibI + ‘lead’ – first letter – of Mystery, meaning intend)

Rub, ail, sit! (3)

ION (printers devilry: RubIO Nails it!)

Stalker’s flag (4)

JACK (double meaning: Jack, as of ‘and the beanstalk’ and ‘union jack’)

Light unit and lead to domain (4)

LAND (‘Light unit’ + AND, meaning domain)

History within toothpaste (4)

PAST (‘within’ reveals ‘toothPASTe’, meaning history)

Exclamation reversed purpose of shelter (5)

HOUSE (‘OH’ reversed + USE (purpose), meaning shelter)

The fir among the trees (5)

Either STONE (printers devilry: The firST ONE among the trees) - suggested by @hdsdv and @JS1 in comments

Ring and veil uncovered reveals actress (5)

OLIVE ('ring' = O + an anagram of 'veil' clued by 'uncovered', i.e. LIVE, giving a female name which can be suggested by 'actress')

Mazed heart before an area (4)

ZONE (‘heart’ as in ‘central letter of’ ‘maZed’ + ONE (an, singular), meaning area)

Chicken! A searching over grass (5)

(best guess at present) PEACE (printer's devilry: Chicken PEA CEase arching over grass) - solved by @JS1 in comments

Settle public awareness (5)

LIGHT (double meaning, as in 'settle', 'land', 'dismount' and also in 'public awareness', 'public knowledge', 'bringing to light') - solved by @JS1 in comments

O pal! Dim tangled qualification (7)

DIPLOMA (anagram of ‘o pal dim’ clued by ‘tangled’, meaning qualification)

Deformed mint gel is changing form (7)

MELTING (anagram of ‘mint gel’ clued by ‘deformed’, meaning changing form)

Taken all together, the 16 solutions then form:

A connecting wall to resolve. The four answers are then connected as a group themselves, giving the final answer to the puzzle.

Resulting solution:

TIME - ZONE (time zone), ON (on time), PAST (past time), UP (time up)
HIGH - AIM (aim high), LAND (highland), DIPLOMA (high diploma), LIGHT (highlight)
POT - ION (potion), JACK (jackpot), MELTING (melting pot), STONE (potstone)

GREEN - OLIVE (olive green), PEACE (Greenpeace), HOUSE (greenhouse), EVA (Eva Green).

The final connection between the four groups is:

TEA - TIME (teatime), HIGH (high tea), POT (teapot), GREEN (green tea).