Doctor, when twitch becomes extreme (7)
I believe the answer is:
drastic
'extreme' is the definition.
(I've seen this before)
'doctor when twitch' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'twitch' becomes 'tic' (tic is a kind of twitch).
'dr'+'as'+'tic'='DRASTIC'
'becomes' acts as a link.
(Other definitions for drastic that I've seen before include "'Dramatic, extreme (7)'" , "Unsparing" , "Uncompromising" , "Extremely severe" , "Radical, extreme" .)