Extreme seeing doctor over a small twitch (7)
I believe the answer is:
drastic
'extreme' is the definition.
(I've seen this before)
'doctor over a small twitch' is the wordplay.
'doctor' becomes 'dr' (synonyms).
'over' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'twitch' becomes 'tic' (tic is a kind of twitch).
'dr'+'a'+'s'+'tic'='DRASTIC'
'seeing' is the link.
(Other definitions for drastic that I've seen before include "'Dramatic, extreme (7)'" , "Extremely severe" , "Grave" , "Uncompromising" , "'Severe, extreme (7)'" .)