In here, sack of greenery! (7)
I believe the answer is:
herbage
'greenery' is the definition.
The answer and definition can be both plants as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'in here sack' is the wordplay.
'in' indicates putting letters inside.
'sack' becomes 'bag' (I've seen this before).
'here' placed around 'bag' is 'HERBAGE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for herbage that I've seen before include "Vegetation" , "Plant life" .)