Partner dispatched cheeky ace (6)
I believe the answer is:
expert
'ace' is the definition.
(both can mean a master of something)
'partner dispatched cheeky' is the wordplay.
'partner dispatched' becomes 'ex' (former partner).
'cheeky' becomes 'pert' (pert can mean cheeky or insolent).
'ex'+'pert'='EXPERT'
(Other definitions for expert that I've seen before include "Person with great knowledge or skill, specialist" , "At the right moment" , "One practised" , "Specialist, pundit" , "Knowledgeable specialist" .)