Bigwig is audibly tense (5)
I believe the answer is:
titan
'bigwig' is the definition.
(I've seen this before)
'audibly tense' is the wordplay.
'audibly' indicates a 'sounds like' (homophone) clue.
'tense' becomes 'tighten' (I've seen this before).
'tighten' sounds like 'TITAN'.
'is' acts as a link.
(Other definitions for titan that I've seen before include "Giant -- rocket" , "Taint in a mythical giant" , "An old Greek god - Atlas was one" , "Hyperion, for example" , "Taint in old Greek god" .)