A cross on island making a self-evident statement (5)
I believe the answer is:
axiom
'a self-evident statement' is the definition.
(I've seen this in another clue)
'a cross on island' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'on' means one lot of letters go next to another.
'island' becomes 'iom' (abbreviation for Isle of Man).
'a'+'x'+'iom'='AXIOM'
'making' acts as a link.
(Other definitions for axiom that I've seen before include "Maxim, self-evident truth" , "Clearly true statement" , "Self-evident statement" , "that's self-evident" , "Universally accepted proposition" .)