Fleece about to warm up (5)
I believe the answer is:
cheat
'fleece' is the definition.
(fleece can mean to cheat someone)
'about to warm up' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to warm up' becomes 'heat' (I've seen this before).
'c'+'heat'='CHEAT'
(Other definitions for cheat that I've seen before include "Trick or swindle" , "Hoodwink" , "rook" , "offending pupil" , "Take unfair advantage of" .)