Terrestrial grounding has lunar beginning (9)
I believe the answer is:
earthling
'terrestrial' is the definition.
(thesaurus)
'grounding has lunar beginning' is the wordplay.
'grounding' becomes 'earthing' (synonyms).
'has' is an insertion indicator.
'beginning' suggests taking the first letters.
The first letter of 'lunar' is 'l'.
'earthing' placed around 'l' is 'EARTHLING'.
(Other definitions for earthling that I've seen before include "sci-fi character" , "Human being (sci-fi)" , "Man of the world" , "man so to be called from space?" .)