Doctor, when facing twitching, is severe (7)
I believe the answer is:
drastic
'severe' is the definition.
(I've seen this before)
'doctor when facing twitching' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'facing' is a charade indicator (letters next to each other).
'twitching' becomes 'tic' (tic is a kind of twitching).
'dr'+'as'+'tic'='DRASTIC'
'is' is the link.
(Other definitions for drastic that I've seen before include "Extreme - forceful" , "Very severe or serious" , "Having strong effect" , "Severe, radical (measures)" , "Unsparing" .)