Second mate's at sea in boat (9)
I believe the answer is:
steamship
'boat' is the definition.
(type of ship)
'second mate's at sea in' is the wordplay.
'second' becomes 's' (common abbreviation).
'at sea' is an anagram indicator.
'in' becomes 'hip' (both can mean fashionable).
'mates' with letters rearranged gives 'teams'.
's'+'teams'+'hip'='STEAMSHIP'
(Other definitions for steamship that I've seen before include "Vessel with funnels" , "What SS stands for" , "Tramp, say" , "I pass them (anag) - vessel" , "SS in full" .)