But one could be in a rat race! (5)
I believe the answer is:
human
'be in a rat race' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'but one' is the wordplay.
'but' becomes 'hum' (I can't explain this - if you can you should give a lot more credence to this answer).
'one' becomes 'an' (I've seen this in other clues).
'hum'+'an'='HUMAN'
'could' acts as a link.
Can you help me to learn more?
(Other definitions for human that I've seen before include "Being that's apt to err?" , "Like people" , "Not divine, animal or mechanical" , "likely to err?" , "Mortal; compassionate" .)