Heedful of a green hollow in which day is advancing (9)
I believe the answer is:
advertent
'heedful of a green hollow in which day is advancing' is the definition.
'advertent' can be an answer for 'heedful' (advertent can mean heedful or attentive). I'm unsure of the rest of the definition.
'heedful of a green hollow in' is the wordplay.
'heedful of' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'a green' becomes 'vert' (I've seen this before).
'hollow' becomes 'dent' (both can mean an impression or indentation).
'in' indicates putting letters inside.
'vert' inserted into 'dent' is 'dvertent'.
'a'+'dvertent'='ADVERTENT'
(Other definitions for advertent that I've seen before include "Taking care" , "Attentive, heedful" .)