Struggles on trendy vessel (10)
I believe the answer is:
battleship
'vessel' is the definition.
(I know that battleship is a type of warship)
'struggles on trendy' is the wordplay.
'struggles' becomes 'battles' ('battle' can be a synonym of 'struggle').
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'trendy' becomes 'hip' (both can mean fashionable).
'battles'+'hip'='BATTLESHIP'
(Other definitions for battleship that I've seen before include "craft for action" , "Large naval vessel" , "Hood, say" , "fighting vessel" , "naval craft" .)