Against the rules for a soldier, I replaced top first (9)
I believe the answer is:
irregular
'against the rules' is the definition.
(I've seen this before)
'soldier i replaced top first' is the wordplay.
'soldier' becomes 'regular' (member of the regular army).
'top' says to take the initial letters (in a down clue, the first letter is top).
'first' is a charade indicator (letters next to each other).
The initial letter of 'replaced' is 'r'.
'i'+'r'='ir'
'regular' after 'ir' is 'IRREGULAR'.
'for a' acts as a link.
(Other definitions for irregular that I've seen before include "Not even or balanced" , "Uneven - inappropriate" , "Rough - guerrilla" , "Guerrilla fighter; not even" , "unusual for a > partisan?" .)