Fierce competition in the 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 believe this answer much more).
'rat'+'race'='RAT-RACE'
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for rat race that I've seen before include "The struggle for survival" , "Competitive way of life" , "day-to-day struggle" , "Desert people in" , "Continual routine of highly competitive activity" .)