Best expose cheat (5)
I believe the answer is:
outdo
'best' is the definition.
('outdo' can be a synonym of 'best')
'expose cheat' is the wordplay.
'expose' becomes 'out' (outing is a kind of exposing).
'cheat' becomes 'do'.
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Get the better of" , "Overtake" , "Overcome" , "going to top?" , "Cap" .)