A couple in old vessel, busy (2,4)
I believe the answer is:
at work
'busy' is the definition.
(similar in meaning)
'a couple in old vessel' is the wordplay.
'a couple' becomes 'two' (a couple can mean two of something).
'in' is an insertion indicator.
'old vessel' becomes 'ark' (I've seen this before).
'two' placed inside 'ark' is 'AT WORK'.
(Other definitions for at work that I've seen before include "Engaged in one's duties" , "busy" , "Active" .)