Large boat: vessel extending across a river (9)
I believe the answer is:
catamaran
'large boat' is the definition.
(type of large boat)
'vessel extending across a river' is the wordplay.
'vessel' becomes 'can' (type of container).
'extending across' is an insertion indicator (some letters extend around others).
'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 "Boat with twin, parallel hulls" , "Vessel with paralleltwin hulls" , "Twin-hulled vessel" , "Quarrelsome woman" , "raft of logs" .)