Attack a vessel having trouble (6)
I believe the answer is:
assail
'attack' is the definition.
('assail' can be a synonym of 'attack')
'a vessel having trouble' is the wordplay.
'vessel' becomes 'ss'.
'having' is a charade indicator (letters next to each other).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'a'+'ss'+'ail'='ASSAIL'
(Other definitions for assail that I've seen before include "Slate" , "Beset" , "Make violent attack on" , "Molest or harass" , "Launch an attack" .)