Crow seen at first in vessel (5)
I believe the answer is:
boast
'crow' is the definition.
(I know that crow is a kind of boast)
'seen at first in vessel' is the wordplay.
'at first' says to take the initial letters.
'in' means one lot of letters goes inside another.
'vessel' becomes 'boat' (boat is a kind of vessel).
The first letter of 'seen' is 's'.
's' inserted into 'boat' is 'BOAST'.
(Other definitions for boast that I've seen before include "Blow one's own trumpet" , "Talk with self-pride" , "Show off" , "Talk with excessive pride" , "shot off the side wall" .)