Dealer Gareth, surprisingly generous (5-7)
I believe the answer is:
large-hearted
'generous' is the definition.
(I've seen this before)
'dealer gareth surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'dealer'+'gareth'='dealergareth'
'dealergareth' with letters rearranged gives 'LARGE-HEARTED'.
(Other definitions for large-hearted that I've seen before include "Of liberal disposition" , "Generous" .)