Gentle spirit coming over in mountain (6)
I believe the answer is:
benign
'gentle' is the definition.
(thesaurus)
'spirit coming over in mountain' is the wordplay.
'spirit' becomes 'gin' (I've seen this before).
'coming over' says the letters should be written in reverse.
'in' is an insertion indicator.
'mountain' becomes 'ben' (ben is a kind of mountain**).
'gin' in reverse letter order is 'nig'.
'nig' placed within 'ben' is 'BENIGN'.
(Other definitions for benign that I've seen before include "Medically curable" , "Obliging" , "Kindly or not malignant" , "Genial; not harmful" , "sweet" .)