Truth is, doctor turned, following a cricket team (5)
I believe the answer is:
axiom
'truth' is the definition.
(I've seen this before)
'doctor turned following a cricket team' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'turned' is a reversal indicator.
'following' says to put letters next to each other.
'cricket team' becomes 'XI' (a cricket team, from Roman numerals for mm).
'mo' reversed gives 'om'.
'a'+'xi'='axi'
'om' after 'axi' is 'AXIOM'.
'is' acts as a link.
(Other definitions for axiom that I've seen before include "Proposition assumed to be self-evident" , "that's self-evident" , "A saying that's widely accepted as true" , "Generally accepted truism" , "Established principle" .)