Honour a team's first principle (5)
I believe the answer is:
axiom
'principle' is the definition.
(an axiom is a belief or principle)
'honour a team's first' is the wordplay.
'honour' becomes 'om'.
'team' becomes 'XI' (a football team, from Roman numerals for 11).
'first' means one lot of letters go next to another.
'a'+'xi'='axi'
'om' after 'axi' is 'AXIOM'.
(Other definitions for axiom that I've seen before include "Dictum" , "truthful statement" , "Self-evident principle" , "Universally accepted proposition" , "Fundamental proposition" .)