Crossword Genius

Mum's pet dog (7)

Ross

I believe the answer is:

mastiff

'dog' is the definition.
(mastiff is a kind of dog)

'mum's pet' is the wordplay.
'mum's' becomes 'ma's' (ma can mean mother).
'pet' becomes 'tiff' (I've seen this in other clues).
'mas'+'tiff'='MASTIFF'

(Other definitions for mastiff that I've seen before include "Powerful dog breed" , "Big dog" , "Large short-haired dog breed" , "Large, powerful, short-haired dog" , "Hound" .)

I've seen this clue in The Sun.
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