Rookie in river, grabbing floating platform (7)
I believe the answer is:
draftee
'rookie' is the definition.
(I've seen this before)
'river grabbing floating platform' is the wordplay.
'river' becomes 'dee' (River Dee).
'grabbing' means one lot of letters goes inside another.
'floating platform' becomes 'raft' (I've seen this before).
'dee' going around 'raft' is 'DRAFTEE'.
'in' is the link.
(Other definitions for draftee that I've seen before include "one brought into service" , "US conscript" , "American military conscript" , "Rookie" , "Soldier, perhaps unwilling" .)