Wrecker prepared to use bar (8)
I believe the answer is:
saboteur
'wrecker' is the definition.
(I know that wrecker can be written as saboteur)
'prepared to use bar' is the wordplay.
'prepared' is an anagram indicator.
'to'+'use'+'bar'='tousebar'
'tousebar' with letters rearranged gives 'SABOTEUR'.
(Other definitions for saboteur that I've seen before include "Oen who causes industrial damage for pay" , "Rue boast about one who commits industrial espionage" , "Abort use (anag)" , "Our beats (anag) - fifth columnist" , "He destroys in order to subvert" .)