Reason some complain ferociously (5)
I believe the answer is:
infer
'reason' is the definition.
(inferring is a kind of reasoning)
'some complain ferociously' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'INFER' is hidden in the letters of 'complain ferociously'.
(Other definitions for infer that I've seen before include "Deduce logically" , "Extrapolate, conclude" , "Take finer meaning from" , "Deduce, surmise" , "Insinuate" .)