Assuming FA backtracking on deal it arranged in league (10)
I believe the answer is:
affiliated
'in league' is the definition.
(thesaurus)
'assuming fa backtracking on deal it arranged' is the wordplay.
'assuming' becomes 'if' (eg 'assuming it's allowed' means 'if it's allowed').
'backtracking' says the letters should be written in reverse.
'on' means one lot of letters goes inside another.
'arranged' indicates anagramming the letters (letters rearranged).
'if'+'fa'='iffa'
'iffa' written backwards gives 'affi'.
'deal'+'it'='dealit'
'dealit' anagrammed gives 'liated'.
'affi' placed into 'liated' is 'AFFILIATED'.
(Other definitions for affiliated that I've seen before include "in partnership" , "adopted" .)