Old ship, new element (5)
I believe the answer is:
argon
'element' is the definition.
(argon is a chemical element)
'old ship new' is the wordplay.
'old ship' becomes 'argo' (I've seen this before).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'argo'+'n'='ARGON'
(Other definitions for argon that I've seen before include "Gas used in electric lights" , "Gaseous element" , "rare gas" , "Inert gas used in light bulbs" , "An insert gas to groan about" .)