Congress assembling, with English pair replaced by American (6)
I believe the answer is:
mating
'congress' is the definition.
(thesaurus)
'assembling with english pair replaced by american' is the wordplay.
I cannot really understand how this works, but
'by' could be 'at' and 'at' is found within the answer.
The remaining letters 'ming' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for mating that I've seen is " Coupling".)