It's mostly busy in the rush hour (3)
I believe the answer is:
bus
'the rush hour' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it's mostly busy' is the wordplay.
'it's mostly' means to remove the last letter (most but not all of the word).
'busy' with its final letter taken away is 'BUS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for bus that I've seen before include "Public service vehicle" , "Road vehicle able to carry many passengers" , "Use transport" , "eg Double-decker" , "Means of public transport" .)