Opposed to increases in a tax, initially (7)
I believe the answer is:
'opposed to' is the definition.
(I've seen this before)
'increases in a tax initially' is the wordplay.
'increases' becomes 'gains' (gain is a kind of increase).
'in' is an insertion indicator.
'initially' suggests taking the first letters.
The first letter of 'tax' is 't'.
'gains' inserted inside 'at' is 'AGAINST'.
(Other definitions for against that I've seen before include "Versus - in contact with" , "Not supporting" , "In opposition to" , "V" , "antagonistic" .)