Give up after tax returns overlooked (9)
I believe the answer is:
forgotten
'overlooked' is the definition.
(thesaurus)
'give up after tax returns' is the wordplay.
'give up' becomes 'forgo' ('forgo' can be a synonym of 'give up').
'after tax' becomes 'nett' (I've seen this before).
'returns' shows that the letters should be reversed in order.
'nett' written backwards gives 'tten'.
'forgo'+'tten'='FORGOTTEN'
(Other definitions for forgotten that I've seen before include "Gone from memory" , "Overlooked" , "Ignored" , "Neglected" , "left behind" .)