Small, trendy vessel (4)
I believe the answer is:
ship
'vessel' is the definition.
(I know that vessel can be written as ship)
'small trendy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trendy' becomes 'hip' (both can mean fashionable).
's'+'hip'='SHIP'
(Other definitions for ship that I've seen before include "Send" , "Sea-going craft" , "part of navy" , "Ocean vessel" , "eg QE2" .)