Bill is a first name (5)
I believe the answer is:
isaac
'name' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I don't see?
'bill is a first' is the wordplay.
'bill' becomes 'ac' (short for account).
'first' is a charade indicator (letters next to each other).
'is'+'a'='isa'
'ac' put after 'isa' is 'ISAAC'.
Can you help me to learn more?
(Other definitions for isaac that I've seen before include "- Newton, Eng. physicist and mathematician" , "one in Genesis" , "Jewish name?" , "Son prepared for sacrifice" , "man whose sons bargained over 3 [A MESS OF POTAGE]" .)