Doctor act is terribly serious (7)
I believe the answer is:
drastic
'serious' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'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'
Can you help me to learn more?
(Other definitions for drastic that I've seen before include "Grave" , "Having strong effect" , "'Severe, extreme (7)'" , "Radical, brutal" , "Extreme - violent" .)