National Insurance in tax year is something worthless (6)
I believe the answer is:
Here is my best explanation:
'something worthless' is the definition.
'national insurance in tax year' is the subsidiary indication.
'national insurance' becomes 'ni' (UK tax).
'in' means one lot of letters goes inside another.
'tax' becomes 'vat' (British sales tax).
'year' becomes 'y'.
'ni' placed inside 'vat' is 'vanit'.
'is' is the link.
(Other definitions for vanity that I've seen before include "Conceitedness, self-love", "Conceit especially about appearance", "Excessive pride in one's self", "William Makepeace Thackeray wrote '. . . . . . Fair'", "Conceit about one's appearance", "Excessive conceit", "Capacity for conceit", "Narcissism", "Futility; conceit" and "Book by Thackeray".)