It's a much shorter distance without the last quarter (4)
I believe the answer is:
mile
'quarter' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'it's a much shorter distance without the last' is the wordplay.
'a much' means to remove the last letter (much but not all of the word).
'shorter distance' becomes 'ml' (I can't explain this - if you can you should give a lot more credence to this answer).
'without' is an insertion indicator.
'last' says to take the final letters.
The last letter of 'the' is 'e'.
'it' with its final letter taken away is 'i'.
'i' put inside 'ml' is 'mil'.
'mil'+'e'='MILE'
Can you help me to learn more?
(Other definitions for mile that I've seen before include "'A miss is as good as a ....'" , "Eight furlongs" , "Imperial length" , "Unit of distance" , "Stretch of road" .)