Crossword Genius

The coalminer's dog, Rex (7)

Ross

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" .)

I've seen this clue in The Telegraph.
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