Sarah's boy is a Bill (5)
I believe the answer is:
isaac
'sarah's boy' is the definition.
(son of Sarah in the Bible)
'is a bill' is the wordplay.
'bill' becomes 'ac' (short for account).
'is'+'a'+'ac'='ISAAC'
(Other definitions for isaac that I've seen before include "Actor Oscar" , "one in Genesis" , "Father to Jacob and Esau" , "one almost sacrificed" , "Sarah's son" .)