Rescue vessel, if able to, at sea (8)
I believe the answer is:
lifeboat
'rescue vessel' is the definition.
(boat which rescues people)
'if able to at sea' is the wordplay.
'at sea' is an anagram indicator.
'if'+'able'+'to'='ifableto'
'ifableto' is an anagram of 'LIFEBOAT'.
(Other definitions for lifeboat that I've seen before include "Vessel to rescue people at sea" , "saver, in the main" , "Small vessel designed to help people at sea" , "It's used to rescue people at sea" , "I bet loaf on maritime rescue vessel" .)