Leading vessel sinks with it (8)
I believe the answer is:
flagship
'leading vessel' is the definition.
'flagship' can be an answer for 'vessel' (flagship is a kind of vessel). I'm not sure about the 'leading' bit.
'sinks with it' is the wordplay.
'sinks' becomes 'flags' (flagging is a kind of sinking).
'with it' becomes 'hip' (both can mean cool or fashionable).
'flags'+'hip'='FLAGSHIP'
Can you help me to learn more?
(Other definitions for flagship that I've seen before include "Admiral's craft in a fleet" , "Most important part of a group" , "It carries the commander of the fleet" , "Pre-eminent item" , "Victory, perhaps" .)