Crossword Genius

Great quantity one floats in river (4)

Ross

I believe the answer is:

raft

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Sun.
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