Dull sweetheart for partner (4)
I believe the answer is:
mate
'partner' is the definition.
(synonyms)
'dull sweetheart' is the wordplay.
'dull' becomes 'mat' (mat as opposed to glossy).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'mat'+'e'='MATE'
'for' is the link.
(Other definitions for mate that I've seen before include "First officer" , "provides solution to chess problem" , "Officer below master on a commercial ship" , "Chess ending; pal" , "Pal, chum" .)