Carp, the sixth to be found in shrunken state (5)
I believe the answer is:
cavil
'carp' is the definition.
(synonyms)
'the sixth to be found in shrunken state' is the wordplay.
'the sixth' becomes 'vi' (Roman numerals).
'to be found in' is an insertion indicator.
'shrunken state' becomes 'cal' (I've seen this in other clues. I am not sure about the 'shrunken' bit.).
'vi' put inside 'cal' is 'CAVIL'.
(Other definitions for cavil that I've seen before include "Raise petty objections" , "Minor complaint" , "Make petty objections" , "Complain" , "Use false arguments" .)