Having a load of cash to underpin impressive hand (5,5)
I believe the answer is:
royal flush
'hand' is the definition.
(I know that royal flush is a hand)
'having a load of cash to underpin impressive' is the wordplay.
'having a load of cash' becomes 'flush' (flush can mean rich).
'to underpin' says to put letters next to each other (in a down clue, some letters go under others).
'impressive' becomes 'royal'.
'flush' after 'royal' is 'ROYAL FLUSH'.
(Other definitions for royal flush that I've seen before include "embarrassment at the palace?" , "making monarch look embarrassed?" , "evidence that monarch occupied throne?" , "Result of embarrassed Queen" , "pulling of chain having vacated throne?" .)