Under Scots mountain hate festering (7)
I believe the answer is:
beneath
'under' is the definition.
(thesaurus)
'scots mountain hate festering' is the wordplay.
'scots mountain' becomes 'ben' (eg Ben Nevis).
'festering' indicates anagramming the letters.
'hate' is an anagram of 'eath'.
'ben'+'eath'='BENEATH'
(Other definitions for beneath that I've seen before include "At a lower place" , "in a lower position" , "Unbefitting" , "Inferior to" , "At a lower level (than)" .)