Attack while yacht maybe goes under (6)
I believe the answer is:
assail
'attack' is the definition.
('assail' can be a synonym of 'attack')
'while yacht maybe goes under' is the wordplay.
'while' becomes 'as'.
'yacht maybe' becomes 'sail'.
'goes under' says to put letters next to each other.
'as'+'sail'='ASSAIL'
(Other definitions for assail that I've seen before include "Attack violently and persistently" , "Trouble" , "Launch an attack" , "Go for" , "Aggressively attack" .)