Prodigal son to hang former head of state (7)
I believe the answer is:
spender
'prodigal' is the definition.
(I know that spender is a type of prodigal)
'son to hang former head of state' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'to hang' becomes 'pend' (I've seen this before).
'former head of state' becomes 'ER' (Elizabeth Regina, ie Queen Elizabeth).
's'+'pend'+'er'='SPENDER'
(Other definitions for spender that I've seen before include "patronising retailers, perhaps?" , "writer of poetry" , "One using money" , "who is just 31[WINDOW SHOPPER]? No" , "Sir Stephen -, English twentieth century poet" .)