Boozer in almost entirely prime colours (7)
I believe the answer is:
blushes
'colours' is the definition.
(blushing is a kind of colouring)
'boozer in almost entirely prime' is the wordplay.
'boozer' becomes 'lush' ('lush' can be a synonym of 'boozer').
'in' is an insertion indicator.
'almost entirely' means to remove the last letter.
'prime' becomes 'best'.
'best' with its final letter removed is 'bes'.
'lush' placed within 'bes' is 'BLUSHES'.
(Other definitions for blushes that I've seen before include "Reddenings of the face, in embarrassment perhaps" , "Turns red with embarrassment" .)