It's bitter always in the south-east (6)
I believe the answer is:
severe
'it's bitter' is the definition.
(thesaurus)
'always in the south-east' is the wordplay.
'always' becomes 'ever' (synonyms).
'in the' means one lot of letters goes inside another.
'south east' becomes 'SE' (abbreviation).
'ever' inserted within 'se' is 'SEVERE'.
(Other definitions for severe that I've seen before include "Austere or strict" , "Harsh and stringent" , "Stern or stringent" , "Harsh and unbending" , "Very plain in style or appearance" .)