Dreaming about sheets? (4)
I believe the answer is:
ream
'sheets' is the definition.
(I've seen this before)
'dreaming about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'REAM' can be found hidden inside 'dreaming'.
(Other definitions for ream that I've seen before include "Lots of paper" , "20 quires of paper" , "480 or 500 sheets" , "Widen out (bore)" , "Make hole larger - lot of paper" .)