It is said that iron fusion is endothermic and star can't sustain this kind of fusion (not until it goes supernova). However star is constantly releasing energy from fusion of elements like Hydrogen and Helium. So, can't that energy be used for fusion of Iron nuclei?
Answer
The Sun obviously produces far more energy per second than is required to fuse an iron nucleus with some other nucleus. The problem is concentrating all that energy on the iron nucleus. It's not enough to know that it takes the energy from $n$ hydrogen fusions to fuse one iron nucleus, it's getting the energetic products from those $n$ hydrogen fusion events to all collide with the iron nucleus at the same time. Under normal conditions the probability of this is negligible.
However, under extreme conditions it can occur. For example in supernovae the pressures and temperatures are so high that iron and heavier nuclei undergo fusion reactions to produce the elements heavier than iron.
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