Male opponents involved in a welcome contract (9)
I believe the answer is:
agreement
'contract' is the definition.
(a contract is a legal agreement)
'male opponents involved in a welcome' is the wordplay.
'male opponents' becomes 'men' (I can't explain this - if you can you should give a lot more credence to this answer).
'involved in' is an insertion indicator.
'welcome' becomes 'greet' (I've seen this in other clues).
'a'+'greet'='agreet'
'men' put inside 'agreet' is 'AGREEMENT'.
Can you help me to learn more?
(Other definitions for agreement that I've seen before include "Union < matching" , "in sympathy" , "Concord, harmony" , "Accord, contract" , "State of being consistent" .)