Understood grapes replaced diamonds (7)
I believe the answer is:
grasped
'understood' is the definition.
(grasping is a kind of understanding)
'grapes replaced diamonds' is the wordplay.
'replaced' indicates an anagram.
'diamonds' becomes 'd' (abbreviation in bridge say**).
'grapes' anagrammed gives 'graspe'.
'graspe'+'d'='GRASPED'
(Other definitions for grasped that I've seen before include "Held firmly or got the meaning" , "Got the picture" , "Understood" , "Took hold of or got the meaning" .)