We're all in this race (5)
I believe the answer is:
human
'we're all in this race' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'in this race' is the wordplay.
'in this' becomes 'hu' (I can't justify this - if you can you should give a lot more credence to this answer).
'race' becomes 'man' (I've seen this before).
'hu'+'man'='HUMAN'
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" , "Anthropoid" , "Not divine, animal or mechanical" , "Like a real person" .)