Doctor registered advantage before ship (7)
I believe the answer is:
dredger
'ship' is the definition.
(I've seen this before)
'doctor registered advantage before' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'registered' becomes 'R' (abbreviation, as in registered trademark).
'advantage' becomes 'edge' (having the edge is having an advantage).
'before' means one lot of letters go next to another.
'r' put after 'edge' is 'edger'.
'dr'+'edger'='DREDGER'
(Other definitions for dredger that I've seen before include "Boat used in scooping out mud, etc., from the bottom of a river" , "Vessel used to deepen a channel" , "Barge-like vessel" , "Vessel for clearing river bed" , "Vessel; sprinkler" .)