It's severe when the south wind goes to the east (7)
I believe the answer is:
austere
'it's severe' is the definition.
(synonyms)
'the south wind goes to the east' is the wordplay.
'the south wind' becomes 'auster' (I've seen this before).
'goes to' says to put letters next to each other.
'the east' becomes 'E' (abbreviation).
'auster'+'e'='AUSTERE'
'when' is the link.
(Other definitions for austere that I've seen before include "Grim - USA tree (anag)" , "Severe, ascetic" , "Severe or strict in attitude" , "Severe; lacking luxuries" , "severely self-disciplined" .)