Saw American chopper landing on island (5)
I believe the answer is:
axiom
'saw' is the definition.
(I've seen this before)
'american chopper landing on island' is the wordplay.
'american chopper' becomes 'ax' (I am not sure about the 'american' bit.).
'landing on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'island' becomes 'iom' (abbreviation for Isle of Man).
'ax'+'iom'='AXIOM'
Can you help me to learn more?
(Other definitions for axiom that I've seen before include "that's self-evident" , "Truth assumed to be self-evident" , "Maxim - truism" , "Truism - proverb" , "Accepted statement taken as self-evidently true" .)