Whale swallowing old boat (5)
I believe the answer is:
canoe
'boat' is the definition.
(canoe is a kind of boat)
'whale swallowing old' is the wordplay.
'whale' becomes 'cane' (both can mean to beat or thrash).
'swallowing' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cane' going around 'o' is 'CANOE'.
(Other definitions for canoe that I've seen before include "Primitive boat" , "Light paddle-propelled boat" , "Light boat propelled by paddle(s)" , "Narrow paddle boat" , "River craft" .)