Conscript's vessel found in river (7)
I believe the answer is:
draftee
'conscript's' is the definition.
(synonyms)
'vessel found in river' is the wordplay.
'vessel' becomes 'raft' (I've seen this before).
'found in' indicates putting letters inside.
'river' becomes 'Dee' (River Dee).
'raft' put inside 'dee' is 'DRAFTEE'.
(Other definitions for draftee that I've seen before include "One forced to join" , "American military conscript" , "US conscript" , "one brought into service" , "Rookie" .)