Crumbs left aboard super boat (7)
I believe the answer is:
coracle
'boat' is the definition.
(coracle is a kind of boat)
'crumbs left aboard super' is the wordplay.
'crumbs' becomes 'cor' (both can be exclamations of shock).
'left' becomes 'l' (common abbreviation).
'aboard' means one lot of letters goes inside another.
'super' becomes 'ace' (synonyms).
'l' put inside 'ace' is 'acle'.
'cor'+'acle'='CORACLE'
(Other definitions for coracle that I've seen before include "Traditional rounded boat with wicker frame" , "Small wickerwork boat" , "Oval rowing boat" , "craft" , "Small primitive boat" .)