Imagine old man stealing unknown quantity of paper (8)
I believe the answer is:
daydream
'imagine' is the definition.
(I know this)
'old man stealing unknown quantity of paper' is the wordplay.
'old man' becomes 'dad' (synonyms).
'stealing' means one lot of letters goes inside another.
'unknown' becomes 'y' (algebra).
'quantity of paper' becomes 'ream' (a ream is a large amount of paper).
'dad' enclosing 'y' is 'dayd'.
'dayd'+'ream'='DAYDREAM'
(Other definitions for daydream that I've seen before include "Indulge in a fantasy" , "Diurnal reveries" , "A pleasant diurnal fantasy" , "Diurnal reverie" , "Wool-gather" .)