Going to ground, having left human (9)
I believe the answer is:
earthling
'human' is the definition.
(earthling is a kind of human)
'going to ground having left' is the wordplay.
'going to ground' becomes 'earthing' (I've seen this before).
'having' is an insertion indicator.
'left' becomes 'l' (common abbreviation).
'earthing' placed around 'l' is 'EARTHLING'.
(Other definitions for earthling that I've seen before include "sci-fi character" , "man so to be called from space?" , "Man of the world" , "Human being (sci-fi)" .)