Young attendant annually emptied greenhouse (4)
I believe the answer is:
page
'young attendant' is the definition.
(I've seen this before)
'annually emptied greenhouse' is the wordplay.
'annually' becomes 'pa' ('p.a.' can be a synonym of 'annually').
'emptied' means to remove the middle letters.
'greenhouse' with its middle removed is 'ge'.
'pa'+'ge'='PAGE'
(Other definitions for page that I've seen before include "Leaf - boy attendant" , "Piece of paper for a young attendant" , "Attendant - sheet" , "contribution to book?" , "Attendant - this is on one!" .)