Almost nude, caught in transgression, gets rash about hidden danger (5,2,3,5)
I believe the answer is:
snake in the grass
'hidden danger' is the definition.
(I've seen this before)
'almost nude caught in transgression gets rash about' is the wordplay.
'almost nude' becomes 'nake' (I can't explain this - if you can you should believe this answer much more).
'caught in' is an insertion indicator.
'transgression' becomes 'sin' (sin is a kind of transgression).
'about' is an anagram indicator.
'gets'+'rash'='getsrash'
'getsrash' is an anagram of 'thegrass'.
'sin'+'thegrass'='sinthegrass'
'nake' put within 'sinthegrass' is 'SNAKE IN THE GRASS'.
(Other definitions for snake in the grass that I've seen before include "unsuspected danger" , "nasty person who turns out to be traitor" , "Treacherous person" , "Hidden danger" , "Judas?" .)