Immoral acts involving grand gestures (5)
I believe the answer is:
signs
'gestures' is the definition.
(signing is a kind of gesturing)
'immoral acts involving grand' is the wordplay.
'immoral acts' becomes 'sins'.
'involving' indicates putting letters inside.
'grand' becomes 'G' (synonyms).
'sins' placed around 'g' is 'SIGNS'.
(Other definitions for signs that I've seen before include "Gestures or symbols" , "Badges" , "Autographs" , "Endorses" , "Signals" .)