Saw a cross on small island (5)
I believe the answer is:
axiom
'saw' is the definition.
(saw can mean a saying or idea)
'a cross on small island' is the wordplay.
'cross' becomes 'x' (an X is a cross).
'on' says to put letters next to each other.
'small island' becomes 'iom' (abbreviation for Isle of Man; 'small' indicates an abbreviation).
'a'+'x'+'iom'='AXIOM'
(Other definitions for axiom that I've seen before include "Established principle" , "Generally accepted truism" , "A saying that's widely accepted as true" , "Generally accepted principle" , "Clearly true statement" .)