Become limp as inferior diamonds rejected (5)
I believe the answer is:
droop
'become limp' is the definition.
('droop' can be a synonym of 'become limp')
'inferior diamonds rejected' is the wordplay.
'inferior' becomes 'poor' ('poor' can be similar in meaning to 'inferior').
'diamonds' becomes 'd' (abbreviation in bridge say).
'rejected' shows that the letters should be reversed in order.
'poor'+'d'='poord'
'poord' written backwards gives 'DROOP'.
'as' acts as a link.
(Other definitions for droop that I've seen before include "be despondent" , "Sag, hang down" , "Become limp" , "lose heart" , "Hang down loosely" .)