Not for profit in promoting The Salvation Army at first (7)
I believe the answer is:
against
'not for' is the definition.
(I've seen this in another clue)
'profit in promoting the salvation army at first' is the wordplay.
'profit' becomes 'gain' (synonyms).
'in' means one lot of letters goes inside another.
'promoting' says the letters should be written backwards (I've seen 'promoted' mean this (going up, in a down clue)).
'at first' suggests taking the first letters.
The first letters of 'the salvation army' is 'tsa'.
'tsa' reversed gives 'ast'.
'gain' inserted inside 'ast' is 'AGAINST'.
(Other definitions for against that I've seen before include "In opposition to" , "Placed in direct contact with" , "on the other side" , "Not favouring" , "Facing" .)