Circular part of drill used in test (7)
I believe the answer is:
orbital
'circular' is the definition.
(thesaurus)
'part of drill used in test' is the wordplay.
'part of drill' becomes 'bit' ('bit' is part of 'drill').
'used in' is an insertion indicator.
'test' becomes 'oral' (oral is a type of examination).
'bit' placed inside 'oral' is 'ORBITAL'.
(Other definitions for orbital that I've seen before include "Gyratory" , "Revolutionary" , "Rob tail (anag.)" , "that's going around" , "kind of motorway" .)