Adult kiss on island revealing truth (5)
I believe the answer is:
axiom
'truth' is the definition.
(I've seen this before)
'adult kiss on island' is the wordplay.
'adult' becomes 'a' (abbreviation).
'kiss' becomes 'x' (X can represent a kiss).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'island' becomes 'iom' (abbreviation for Isle of Man).
'a'+'x'+'iom'='AXIOM'
'revealing' is the link.
(Other definitions for axiom that I've seen before include "Truism - proverb" , "truthful statement" , "Acceptable principle" , "Old saying or precept 95)" , "Universal principle" .)