Cheat vulnerable competitor (9)
I believe the answer is:
contender
'competitor' is the definition.
('contender' can be a synonym of 'competitor')
'cheat vulnerable' is the wordplay.
'cheat' becomes 'con' (con is a kind of cheat).
'vulnerable' becomes 'tender' ('tender' can be a synonym of 'vulnerable').
'con'+'tender'='CONTENDER'
(Other definitions for contender that I've seen before include "One struggling" , "Trend once (anag.)" , "One fighting others (for something)" , "One striving" , "One who strives" .)