Move, if able, to rescue vessel (8)
I believe the answer is:
lifeboat
'rescue vessel' is the definition.
(boat which rescues people)
'move if able to' is the wordplay.
'move' is an anagram indicator (I've seen 'moves' mean this).
'if'+'able'+'to'='ifableto'
'ifableto' with letters rearranged gives 'LIFEBOAT'.
(Other definitions for lifeboat that I've seen before include "Rescuer" , "saver, in the main" , "needed if ship sinks" , "Take to it if the ship is sinking" , "To fable I turn if in danger of sinking" .)