Boat can, sheltering at a ruin (9)
I believe the answer is:
catamaran
'boat' is the definition.
(I know that catamaran is a type of boat)
'can sheltering at a ruin' is the wordplay.
'sheltering' indicates putting letters inside.
'ruin' becomes 'mar' (I've seen this before).
'at'+'a'+'mar'='atamar'
'can' placed around 'atamar' is 'CATAMARAN'.
(Other definitions for catamaran that I've seen before include "Twin-hulled sailing boat" , "Two-hulled boat" , "raft of logs" , "Twin-hulled craft" , "Twin-hulled vessel" .)