Mine worker taking sheepdog by river (7)
'mine worker' is the definition.
'sheepdog by river' is the wordplay.
'sheepdog' becomes 'collie' (collie is a kind of sheepdog).
'by' says to put letters next to each other.
'river' becomes 'r'.
'taking' acts as a link.
(Other definitions for collier that I've seen before include "Painter", "Ship carrying coal", "unable to avoid contact with mine", "Pitman", "Miner; coal ship".)