One brought in to launch vessel into river (7)
I believe the answer is:
draftee
'one brought in' is the definition.
(someone drafted in)
'vessel into river' is the wordplay.
'vessel' becomes 'raft' (I've seen this before).
'into' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'raft' put into 'dee' is 'DRAFTEE'.
'to launch' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for draftee that I've seen before include "American military conscript" , "Soldier, perhaps unwilling" , "One forced to join" , "Rookie" , "US conscript" .)