Crossword Genius

Tom, right on the tail of his victim? (6)

Ross

I believe the answer is:

mouser

I'm a little stuck... Click here to teach me more about this clue!

This clue might be a double definition.

'tom' is the first definition.
(thesaurus)

'right on the tail of his victim?' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

Can you help me to learn more?

(Other definitions for mouser that I've seen before include "Cat proficient at catching small rodents" , "Predatory cat" , "Cat that's good at catching small rodents" , "Cat as hunter of small rodents" , "One getting rid of vermin" .)

I've seen this clue in the Evening Standard.
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