Extreme vessel caught out (5)
I believe the answer is:
utter
'extreme' is the definition.
(I've seen this before)
'vessel caught out' is the wordplay.
'vessel' becomes 'cutter' (cutter is a kind of vessel).
'caught' becomes 'c' (cricket abbreviation).
'out' indicates named letters should be taken away.
'cutter' with 'c' removed is 'UTTER'.
(Other definitions for utter that I've seen before include "in toto" , "Speak - it's extreme" , "State - unqualified - rank" , "To say, articulate" , "Antique offer for sale" .)