A lot of leaves about, arboretum getting emptied (4)
I believe the answer is:
ream
'a lot of leaves' is the definition.
(a ream is 500 sheets or 'leaves' of paper)
'about arboretum getting emptied' is the wordplay.
'about' becomes 're' (regarding).
'getting emptied' means to remove the middle letters (the inside letters are emptied out).
'arboretum' with its middle removed is 'am'.
're'+'am'='REAM'
(Other definitions for ream that I've seen before include "Quantity of paper, about 500 sheets" , "stock of paper" , "500 sheets, perhaps" , "516 sheets of paper" , "papers" .)