Outsider provides rescue vessel, when surrounded by rampaging hordes (4,5)
I believe the answer is:
dark horse
'outsider' is the definition.
'rescue vessel when surrounded by rampaging hordes' is the wordplay.
'rescue vessel' becomes 'ark' (I've seen this before).
'when surrounded by' indicates putting letters inside.
'rampaging' is an anagram indicator (I've seen this in other clues).
'hordes' anagrammed gives 'dhorse'.
'ark' inserted within 'dhorse' is 'DARK HORSE'.
'provides' is the link.
(Other definitions for dark horse that I've seen before include "Unknown quantity" , "Black Beauty, for instance" , "Competitor of unknown capabilities" , "Successful competitor of whom little was known, like colourless equine" , "Brown heroin" .)