Quarter regularly spent on a wee derivative (4)
I believe the answer is:
urea
'wee derivative' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'quarter regularly spent on a' is the wordplay.
'regularly spent' means one should take alternating letters (letters removed at regular intervals).
'on' says to put letters next to each other.
The alternate letters of 'quarter' are 'ure'.
'ure'+'a'='UREA'
Can you help me to learn more?
(Other definitions for urea that I've seen before include "It's produced by piddling" , "wee substance?" , "Compound found in urine, synthesised as a fertiliser" , "Human waste product" , "organic compound" .)