Drastic, thorough (7)
I believe the answer is:
radical
I believe this is a double definition.
'drastic' is the first definition.
(I've seen this before)
'thorough' is the second definition.
(as in a radical or thorough overhaul of something)
(Other definitions for radical that I've seen before include "Progressive; extreme" , "Basic" , "Going to the roots, fundamental as change may be" , "Dial car in order to be drastic" , "One seeking root and branch reform" .)