Boat can, crossing a West Country river (9)
I believe the answer is:
catamaran
'boat' is the definition.
(catamaran is a kind of sailing vessel)
'can crossing a west country river' is the wordplay.
'crossing' is an insertion indicator.
'west country river' becomes 'tamar' (I've seen this before).
'a'+'tamar'='atamar'
'can' enclosing 'atamar' is 'CATAMARAN'.
(Other definitions for catamaran that I've seen before include "Twin-hulled craft" , "Quarrelsome woman" , "Boat with two hulls" , "One vessel" , "Two-hulled boat" .)