Dying - bone buried in hillock (8)
I believe the answer is:
moribund
'dying' is the definition.
(moribund means dying)
'bone buried in hillock' is the wordplay.
'bone' becomes 'rib' (rib is a type of bone).
'buried in' means one lot of letters goes inside another (inserted letters get buried).
'hillock' becomes 'mound' (synonyms).
'rib' inserted inside 'mound' is 'MORIBUND'.
(Other definitions for moribund that I've seen before include "lacking vitality" , "At death's door" , "Decaying" , "not quite dead" , "Stagnant - dying" .)