Covering of grass around open country (9)
I believe the answer is:
sheathing
'covering' is the definition.
(I have seen 'protective covering' mean 'sheathing' so perhaps 'covering' could also mean 'sheathing')
'grass around open country' is the wordplay.
'grass' becomes 'sing' (I've seen this in other clues).
'around' means one lot of letters goes inside another.
'open country' becomes 'heath' (I've seen this before).
'sing' placed around 'heath' is 'SHEATHING'.
'of' is the link.
(Other definitions for sheathing that I've seen before include "protective covering" , "putting it in a case" .)