Vessel's new keel regularly maintained by sailor (6)
I believe the answer is:
tanker
'vessel's' is the definition.
(tanker is a kind of vessel)
'new keel regularly maintained by sailor' is the wordplay.
'new' becomes 'n'.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'maintained by' means one lot of letters goes inside another (maintain can mean to keep hold of).
'sailor' becomes 'tar' (tar is a kind of sailor).
The alternating letters of 'keel' are 'ke'.
'n'+'ke'='nke'
'nke' going inside 'tar' is 'TANKER'.
(Other definitions for tanker that I've seen before include "Petrol transporter" , "Oil-carrying ship" , "Vehicle or ship for carrying oil in bulk" , "Heavy vehicle carrying liquids" , "Oil-carrying vessel" .)