Unexpectedly go to war in British vessel (6,4)
I believe the answer is:
rowing boat
'vessel' is the definition.
(I know that rowing boat is a type of boat)
'unexpectedly go to war in british' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'british' becomes 'b' (abbreviation e.g. in BBC).
'go'+'to'+'war'+'in'+'b'='gotowarinb'
'gotowarinb' with letters rearranged gives 'ROWING-BOAT'.
(Other definitions for rowing boat that I've seen before include "Small vessel propelled by oars" , "Maybe eight" .)