Contract to dispose of diamonds (4)
I believe the answer is:
bind
'contract' is the definition.
(thesaurus)
'dispose of diamonds' is the wordplay.
'dispose of' becomes 'bin' ().
'diamonds' becomes 'd' (abbreviation in card games).
'bin'+'d'='BIND'
'to' acts as a link.
(Other definitions for bind that I've seen before include "Fix" , "Impose a contractual obligation on (someone)" , "Tie; problem situation" , "Tie up; annoyance" , "This weed" .)