The coalminer's dog, Rex (7)
I believe the answer is:
collier
'the coalminer's' is the definition.
(I've seen this before)
'dog rex' is the wordplay.
'dog' becomes 'collie' (collie is a kind of dog).
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'collie'+'r'='COLLIER'
(Other definitions for collier that I've seen before include "Ship carrying coal" , "Pitman" , "British coal-miner" , "He works in a coal mine" , "Cargo ship" .)