Amateur team doctor, retiring, accepted truth (5)
I believe the answer is:
axiom
'accepted truth' is the definition.
(I've seen this in another clue)
'amateur team doctor retiring' is the wordplay.
'amateur' becomes 'a'.
'team' becomes 'XI' (a football team, from Roman numerals for mm).
'doctor' becomes 'mo' (medical officer).
'retiring' says the letters should be written backwards.
'mo' reversed gives 'om'.
'a'+'xi'+'om'='AXIOM'
(Other definitions for axiom that I've seen before include "Universally accepted proposition" , "Proposition assumed to be self-evident" , "Generally accepted principle" , "Acceptable principle" , "Generally accepted truism" .)