Doctor act is terribly severe (7)
I believe the answer is:
drastic
'severe' is the definition.
(I've seen this before)
'doctor act is terribly' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'terribly' indicates an anagram.
'act'+'is'='actis'
'actis' with letters rearranged gives 'astic'.
'dr'+'astic'='DRASTIC'
(Other definitions for drastic that I've seen before include "Very severe or serious" , "Grave" , "Draconian" , "Radical, brutal" , "Harsh, severe" .)