Become dejected when inferior diamonds returned (5)
I believe the answer is:
droop
'become dejected' is the definition.
(droop can mean to sag from dejection)
'inferior diamonds returned' is the wordplay.
'inferior' becomes 'poor' (similar in meaning).
'diamonds' becomes 'd' (abbreviation in bridge say).
'returned' shows that the letters should be reversed in order.
'poor'+'d'='poord'
'poord' in reverse letter order is 'DROOP'.
'when' is the link.
(Other definitions for droop that I've seen before include "Dangle" , "Grow faint" , "male problem?" , "Sag wearily" , "Sag, hang down" .)