Radical doctor in charge of taking all scientific tests at first (7)
I believe the answer is:
drastic
'radical' is the definition.
(drastic can mean radical or extreme)
'doctor in charge of taking all scientific tests at first' is the wordplay.
'doctor' becomes 'dr' (abbreviation).
'in charge of' becomes 'ic'.
'taking' indicates putting letters inside.
'at first' indicates taking the first letters.
The initial letters of 'all scientific tests' is 'ast'.
'dr'+'ic'='dric'
'dric' placed around 'ast' is 'DRASTIC'.
(Other definitions for drastic that I've seen before include "Extreme - violent" , "Unsparing" , "Radical, extreme" , "Extreme - forceful" , "Desperate" .)