A charmer about to be detected in depravity (5)
I believe the answer is:
siren
'a charmer' is the definition.
(I've seen this in another clue)
'about to be detected in depravity' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to be detected in' is an insertion indicator.
'depravity' becomes 'sin' (I've seen this before).
're' inserted within 'sin' is 'SIREN'.
(Other definitions for siren that I've seen before include "Warning device - femme fatale" , "Hooter - dangerous woman?" , "Loud whistle or sailors' enchantress" , "Bewitching singer,part- woman, part-bird" , "Seductress" .)