American petrol mostly cut (3)
I believe the answer is:
gas
'american petrol' is the definition.
(I've seen this before)
'mostly cut' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'cut' becomes 'gash' (a gash is a wound or cut).
'gash' with its last letter taken away is 'GAS'.
(Other definitions for gas that I've seen before include "Possibly noble" , "Nitrogen, say" , "Idle chat" , "may fill American car" , "Fuel; idle talk" .)