story - The Word Web - Clue Sixteen

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_{Inspiration from here.}

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_{A lot of credit to @Scimonster here; we created this puzzle together.}

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"Aha!" you exclaim, finally spotting the door - it's an almost invisible gray door against the wall. You head over there, expecting to find a challenge - and you aren't disappointed. Inscribed above the door is a strange sign:

Next to the door is a plaque. On the plaque it says:

Remember the Clue Web, on your favorite website, posted by the renowned puzzler Brent Hackers? Excellent, because now's your chance to solve one! Maybe you don't remember how to to play. (Even though it was an awesome puzzle.) Mithrandir, here are the rules: Begin by taking one group of four words. Each word in the group can be connected to another word, which is not here. Repeat for the next three groups. Then, you take those four words, and find the word that connects those three. (Hopefully, there will be only one.) After that, you'll have your precious Clue. Terrific, no?

Group 1
gain, fee, no, point
tomato, egg, green, fruit
gong, liberty, Alexander, curve
floor, ballroom, around, modern

Group 2

white, midnight, cat, pitch
lining, bullet, tongue, sterling
key, horns, caps, hair
cross, swallowing, broad, wound

Group 3
red, oven, ton, wall
roll, skipping, baby, chair
zebra, true, earn, stars
know, opinion, matter, ground

Group 4

stroke, pack, scratch, log
side, get, loud, right
fast, thunder, rod, storm
fates, pigs, bears, witches

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Answer

Group 1

bar (@Aethon)
entry gain, fee, no, point (@Sconibulus)
salad tomato, egg, green, fruit (@Sconibulus)
bell gong, liberty, Alexander, curve
dance floor, ballroom, around, modern

Group 2

smith
black white, midnight, cat, pitch

silver lining, bullet, tongue, sterling
lock key, horns, caps, hair
sword cross, swallowing, broad, wound

Group 3

solid
brick red, oven, ton, wall
rock roll, skipping, baby, chair
stripes zebra, true, earn, stars

truth know, opinion, matter, ground

Group 4

strike (@Sconibulus)
back stroke, pack, scratch, log
out side, get, loud, right
lightning fast, thunder, rod, storm
three fates, pigs, bears, witches

And based on work by @Sconibulus and comments by @Aethon, the final group is

gold (@Aethon) (bar, smith, solid, strike)

So the clue is

Gold