Groom about to be put in prison (5)
I believe the answer is:
preen
'groom' is the definition.
(thesaurus)
'about to be put in prison' is the wordplay.
'about' becomes 're' (regarding**).
'to be put in' means one lot of letters goes inside another.
'prison' becomes 'pen' (I've seen this before).
're' going into 'pen' is 'PREEN'.
(Other definitions for preen that I've seen before include "Self-congratulate" , "Groom with great care" , "Vainly groom oneself" , "Dress or smooth, feathers say" , "Tidy/clean (feathers)" .)