Kill American snake beginning to terrorise ship (7)
I believe the answer is:
iceboat
'ship' is the definition.
(iceboat is a kind of ship)
'kill american snake beginning to terrorise' is the wordplay.
'kill american' becomes 'ice' (I've seen this before).
'snake' becomes 'boa' (boa constrictor, type of snake).
'beginning to' says to take the initial letters.
The first letter of 'terrorise' is 't'.
'ice'+'boa'+'t'='ICEBOAT'
(Other definitions for iceboat that I've seen before include "Arctic vessel" , "Ship used to clear way in polar waters" , "it cuts through arctic seas" , "One'll break through the pack" , "Craft" .)