Opposed to a way to secure advantage (7)
I believe the answer is:
against
'opposed to' is the definition.
(I've seen this before)
'a way to secure advantage' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to' says to put letters next to each other (I've seen this in other clues).
'secure advantage' becomes 'gain' (gain is a kind of advantage. I am not sure about the 'secure' bit.).
'st' after 'gain' is 'gainst'.
'a'+'gainst'='AGAINST'
Can you help me to learn more?
(Other definitions for against that I've seen before include "Anti, touching" , "In conflict with" , "V" , "Placed in direct contact with" , "Not favouring" .)