Diamonds, something difficult to beat (4)
I believe the answer is:
drub
'beat' is the definition.
('drub' can be a synonym of 'beat')
'diamonds something' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'something' becomes 'rub' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'd'+'rub'='DRUB'
'difficult to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for drub that I've seen before include "Beat hollow" , "Beat conclusively in a competition or fight" , "Thrash" , "Beat thoroughly" , "Wipe floor with" .)