Old metalworker alternatively employed in seagoing vessel (7)
I believe the answer is:
loriner
'old metalworker' is the definition.
'alternatively employed in seagoing vessel' is the wordplay.
'alternatively' becomes 'or'.
'employed in' means one lot of letters goes inside another.
'seagoing vessel' becomes 'liner'.
'or' inserted within 'liner' is 'LORINER'.
(Other definitions for loriner that I've seen before include "one involved with Spurs" , "One who made a bit" .)