What sleeper does naked in sheets (4)

Ross

I believe the answer is:

ream

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

'in sheets' is the definition.
'ream' can be an answer for 'sheets' (I've seen this before). I'm not sure about the 'in' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ream that I've seen before include "20 quires of paper" , "stock of paper" , "Five hundred sheets of paper" , "Many sheets - enlarge hole" , "Twenty quires of paper" .)

I've seen this clue in The Independent.

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