Former pit worker holding a marker (8)
I believe the answer is:
examiner
'marker' is the definition.
(someone who marks exams)
'former pit worker holding a' is the wordplay.
'former' becomes 'ex'.
'pit worker' becomes 'miner' (I've seen this before).
'holding' is an insertion indicator.
'ex'+'miner'='exminer'
'exminer' placed around 'a' is 'EXAMINER'.
(Other definitions for examiner that I've seen before include "bloke who will make inspection" , "Assessor" , "Test setter" , "Tester of knowledge" , "Meaner XI (anag)" .)