A lot of paper in the preamble (4)
I believe the answer is:
ream
'a lot of paper' is the definition.
'in the preamble' is the wordplay.
'in' says the answer is hidden in the clue.
'REAM' can be found hidden inside 'preamble'.
(Other definitions for ream that I've seen before include "papers" , "500 sheets, perhaps" , "Five hundred sheets of paper" , "Many bits of paper" , "Widen (hole) with borer; twenty quires" .)