Crossword Genius

Fierce competition in rodent population? (3,4)

Ross

I believe the answer is:

rat race

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

'fierce competition' is the definition.

'rodent population?' is the wordplay.
'rodent' becomes 'rat' (rat is a kind of rodent).
'population?' becomes 'race' (I can't explain this - if you can you should give a lot more credence to this answer).
'rat'+'race'='RAT RACE'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for rat race that I've seen before include "Competitive way of life" , "Continual round of hectic competition" , "The struggle to get on in life" , "unhealthy competition" , "Fierce competition to succeed in life" .)

I've seen this clue in The Sun.
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