Saw small island following a cross (5)
I believe the answer is:
axiom
'saw' is the definition.
(saw can mean a saying or idea)
'small island following a cross' is the wordplay.
'small island' becomes 'iom' (abbreviation for Isle of Man; 'small' indicates a short form).
'following' says to put letters next to each other.
'cross' becomes 'x' (an X is a cross).
'a'+'x'='ax'
'iom' after 'ax' is 'AXIOM'.
(Other definitions for axiom that I've seen before include "Acceptable principle" , "fundamental principle" , "Obviously true statement" , "Proposition assumed to be self-evident" , "It's taken for granted" .)