Sheets on early in the day (4)
I believe the answer is:
ream
'sheets' is the definition.
(I've seen this before)
'on early in the day' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'early in the day' becomes 'am' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'am'='REAM'
Can you help me to learn more?
(Other definitions for ream that I've seen before include "Make hole larger - lot of paper" , "480 sheets of paper" , "Quantity of paper, about 500 sheets" , "Widen (hole); quantity of paper" , "Widen (hole) with borer; twenty quires" .)