Partner harbours determination to become artist (8)
I believe the answer is:
magritte
'artist' is the definition.
(Rene Magritte, Belgian painter)
'partner harbours determination' is the wordplay.
'partner' becomes 'mate' (synonyms).
'harbours' indicates putting letters inside.
'determination' becomes 'grit' (grit can mean determination or tenacity).
'mate' enclosing 'grit' is 'MAGRITTE'.
'to become' is the link.
(Other definitions for magritte that I've seen before include "Belgian surrealist artist" , "Rene -, Surrealist painter" , "RenZ -, Belgian artist" , "Belgian surrealist painter, d.1967" , "Belgian surreal painter" .)