Opposed to increases in a tax, initially (7)
I believe the answer is:
against
'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'.
'a'+'t'='at'
'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" .)