National Insurance in tax year is something worthless (6)
I believe the answer is:
vanity
'something worthless' is the definition.
'national insurance in tax year' is the wordplay.
'national insurance' becomes 'ni' (UK tax).
'in' is an insertion indicator.
'tax' becomes 'vat' (British sales tax).
'year' becomes 'y'.
'ni' put inside 'vat' is 'vanit'.
'vanit'+'y'='VANITY'
'is' acts as a link.
(Other definitions for vanity that I've seen before include "Conceit especially about appearance" , "Excessive pride in one's self" , "Excessive conceit" , "Conceit; pointlessness" , "Capacity for conceit" .)