Bill is a leading man (5)
I believe the answer is:
isaac
'man' is the definition.
(I've seen this before)
'bill is a leading' is the wordplay.
'bill' becomes 'ac' (short for account).
'leading' is a charade indicator (letters next to each other).
'is'+'a'='isa'
'ac' put after 'isa' is 'ISAAC'.
(Other definitions for isaac that I've seen before include "Jewish name?" , "Sir -- Newton, English scientist" , "Biblical patriarch" , "Esau's father" , "-- Asimov, science fiction writer" .)