Prodigal son hoarding what's more (4)
I believe the answer is:
also
'what's more' is the definition.
(I've seen this in another clue)
'prodigal son hoarding' is the wordplay.
'hoarding' indicates the answer is hidden within the clue.
'ALSO' is hidden in the letters of 'prodigal son'.
(Other definitions for also that I've seen before include "on top" , "To boot" , "admitted to Royal Society > too" , "Item" , "Additionally" .)