Do up bows of each vessel within slipway (6)
I believe the answer is:
revamp
'do up' is the definition.
(thesaurus)
'bows of each vessel within slipway' is the wordplay.
'bows of' says to take the initial letters (I've seen 'bow of' mean this (front of)).
'within' is an insertion indicator.
'slipway' becomes 'ramp' (a slipway is a ramp by water).
The initial letters of 'each vessel' is 'ev'.
'ev' put within 'ramp' is 'REVAMP'.
(Other definitions for revamp that I've seen before include "Give new form to" , "Make new, improve or up-date" , "New and improved version" , "face-lift" , "Give new appearance to" .)