Pit worker (7)
'pit worker' is the definition.
(miners work down pits)
This is the entire clue.
(Other definitions for miner that I've seen before include "it's the pits, for me" , "Worker below ground" , "Digger for gold or coal, say" , "Bevin boy" , "Underground worker" .)
(Other definitions for collier that I've seen before include "Miner; coal ship" , "Ship carrying coal" , "unable to avoid contact with mine" , "Geordie Jack, commonly" , "Painter" .)