Declined rubbish diamonds (7)
I believe the answer is:
refused
'declined' is the definition.
('refuse' can be a synonym of 'decline')
'rubbish diamonds' is the wordplay.
'rubbish' becomes 'refuse' (both can mean waste material).
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'refuse'+'d'='REFUSED'
(Other definitions for refused that I've seen before include "Said no (7)" , "Declined, rejected" , "Not accepted" , "Was unwilling to accept" , "turned down appeal" .)