Conscript’s floating structure in river (7)
I believe the answer is:
draftee
'conscript's' is the definition.
('draftee' can be a synonym of 'conscript')
'floating structure in river' is the wordplay.
'floating structure' becomes 'raft' (I've seen this before).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'raft' placed within 'dee' is 'DRAFTEE'.
(Other definitions for draftee that I've seen before include "one brought into service" , "Soldier, perhaps unwilling" , "American military conscript" , "Rookie" , "US conscript" .)