Inspector formerly a collier (8)
I believe the answer is:
examiner
'inspector' is the definition.
(I know that inspector can be written as examiner)
'formerly a collier' is the wordplay.
'formerly' becomes 'ex' (prefix meaning former or onetime).
'collier' becomes 'miner' (I've seen this in another clue).
'ex'+'a'+'miner'='EXAMINER'
(Other definitions for examiner that I've seen before include "One setting a formal test" , "Tester of knowledge" , "He gives you paper" , "One administering test" , "bloke who will make inspection" .)