Impound vessel for unpaid work (10)
I believe the answer is:
internship
'unpaid work' is the definition.
(interns are typically unpaid)
'impound vessel' is the wordplay.
'impound' becomes 'intern' (intern can mean to impound or imprison).
'vessel' becomes 'ship' (ship is a kind of vessel).
'intern'+'ship'='INTERNSHIP'
'for' is the link.
(Other definitions for internship that I've seen before include "Trainee's spell" , "Work experience position" , "being a trainee?" , "practical training programme" .)