Small cool vessel... (4)
I believe the answer is:
ship
'vessel' is the definition.
(I know that ship is a type of vessel)
'small cool' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cool' becomes 'hip' (hip can mean cool or fashionable).
's'+'hip'='SHIP'
(Other definitions for ship that I've seen before include "part of navy" , "Large boat, dispatch" , "craft of 1d [ADVENTURE]" , "Ocean vessel" , "Transport as on sea-going vessel" .)