That of the moment? (4)
I believe the answer is:
spur
'moment?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for spur that I've seen before include "Egg on or goad as horseman might" , "Goad, prod" , "footballer?" , "slip road" , "Encourage" .)