Great quantity one floats in river (4)
I believe the answer is:
raft
'one floats in river' is the definition.
I can't judge whether this defines the answer.
'great quantity' is the wordplay.
'great' becomes 'raf' (I am not sure about this - if you are sure you should believe this answer much more).
'quantity' becomes 'T' (T is an example).
'raf'+'t'='RAFT'
Can you help me to learn more?
(Other definitions for raft that I've seen before include "Makeshift boat" , "Flat water craft; great collection" , "Simple craft" , "Wooden craft" , "host" .)