Some paper involved in preamble (4)
I believe the answer is:
ream
'some paper' is the definition.
(a ream is an amount of paper)
'involved in preamble' is the wordplay.
'involved in' indicates the answer is hidden within the clue.
'REAM' can be found hidden inside 'preamble'.
(Other definitions for ream that I've seen before include "Quantity of paper, about 500 sheets" , "papers" , "Many sheets - enlarge hole" , "20 quires of paper" , "Widen out (bore)" .)