Nothing but national insurance money (3)
I believe the answer is:
nil
'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').
'ni'+'l'='NIL'
'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" .)