Resist alien vessel (6)
I believe the answer is:
bucket
'vessel' is the definition.
(bucket is a kind of vessel)
'resist alien' is the wordplay.
'resist' becomes 'buck' (I've seen this before).
'alien' becomes 'et' (extra terrestrial).
'buck'+'et'='BUCKET'
(Other definitions for bucket that I've seen before include "Scuttle" , "Destroy ? dismantle" , "Container for carrying, sand perhaps" , "Waterwheel compartment" , "Water receptacle" .)