Small sides in vessel (9)
I believe the answer is:
steamship
'vessel' is the definition.
(I know that steamship is a type of vessel)
'small sides in' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sides' becomes 'teams' (I've seen this in another clue).
'in' becomes 'hip' (both can mean fashionable).
's'+'teams'+'hip'='STEAMSHIP'
(Other definitions for steamship that I've seen before include "What SS stands for" , "I pass them (anag) - vessel" , "Queen Elizabeth, for one?" , "Tramp, say" , "SS in full" .)