Groom about to enter enclosure (5)
I believe the answer is:
preen
'groom' is the definition.
(a bird grooms itself by preening feathers)
'about to enter enclosure' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to enter' is an insertion indicator.
'enclosure' becomes 'pen' (pen is a kind of enclosure).
're' going into 'pen' is 'PREEN'.
(Other definitions for preen that I've seen before include "setdown straight?" , "Dress or smooth, feathers say" , "Dress with elaborate care" , "Self-congratulate" , "Array (oneself) carefully" .)