Fierce competition in rodent population? (3,4)
I believe the answer is:
rat race
'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" .)