Dig for diamonds (4)
I believe the answer is:
prod
'dig' is the definition.
('prod' can be a synonym of 'dig')
'for diamonds' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'diamonds' becomes 'd' (abbreviation in bridge say).
'pro'+'d'='PROD'
(Other definitions for prod that I've seen before include "Poke - remind" , "Poke or spur" , "Induce action (with stick etc?)" , "Stimulus" , "Incite to action (with a stick?)" .)