A team only Mancunians originally saw? (5)
I believe the answer is:
axiom
'saw?' is the definition.
(thesaurus)
'a team only mancunians originally' is the wordplay.
'team' becomes 'xi' (eleven, a football team).
'originally' says to take the initial letters.
The initial letters of 'only mancunians' is 'om'.
'a'+'xi'+'om'='AXIOM'
(Other definitions for axiom that I've seen before include "Maxim, self-evident truth" , "Truth assumed to be self-evident" , "Generally accepted proposition" , "It's taken for granted" , "Universally accepted proposition" .)