Become dejected when inferior diamonds shown up (5)
I believe the answer is:
droop
'become dejected' is the definition.
(droop can mean to sag from dejection)
'inferior diamonds shown up' is the wordplay.
'inferior' becomes 'poor' (similar in meaning).
'diamonds' becomes 'd' (abbreviation in card games).
'shown up' shows that the letters should be reversed in order (in a down clue, letters go up).
'poor'+'d'='poord'
'poord' written backwards gives 'DROOP'.
'when' acts as a link.
(Other definitions for droop that I've seen before include "sink" , "be despondent" , "male problem?" , "Dangle" , "Become limp" .)