Inspector formerly an underground operative (8)
I believe the answer is:
examiner
'inspector' is the definition.
('examiner' can be a synonym of 'inspector')
'formerly an underground operative' is the wordplay.
'formerly' becomes 'ex'.
'an' becomes 'a'.
'underground operative' becomes 'miner'.
'ex'+'a'+'miner'='EXAMINER'
(Other definitions for examiner that I've seen before include "Meaner XI (anag)" , "One looks for information" , "One administering test" , "One who tests, inspects or inquires into" , "One in charge of test" .)