Nothing but national insurance money (3)
I believe the answer is:
'nothing' is the definition.
(eg nil is zero in football)
'national insurance money' is the wordplay.
'national insurance' becomes 'NI' (UK tax).
'money' becomes 'l' (L can mean 'pounds').
'but' acts as a link.
(Other definitions for nil that I've seen before include "Nothing found in senility" , "Lacking value" , "Nix, nada, nothing" , "Nothing, zero" , "Zero, no score" .)