Man after a kiss in principle (5)
I believe the answer is:
axiom
'principle' is the definition.
(an axiom is a belief or principle)
'man after a kiss' is the wordplay.
'man' becomes 'iom' (abbreviation for Isle of Man).
'after' says to put letters next to each other.
'kiss' becomes 'x' (X can represent a kiss).
'a'+'x'='ax'
'iom' after 'ax' is 'AXIOM'.
'in' acts as a link.
(Other definitions for axiom that I've seen before include "Old saying or precept 95)" , "Self-evident statement" , "that goes without saying" , "Adage" , "Fundamental proposition" .)