After zero, the extent of variation in colour (6)
I believe the answer is:
orange
'variation in colour' is the definition.
'orange' can be an answer for 'colour' (orange is a kind of colour). I'm unsure of the 'variation in' bit.
'after zero the extent' is the wordplay.
'after' means one lot of letters go next to another.
'zero' becomes 'O' (O resembles 0).
'the extent' becomes 'range' (range is a kind of extent).
'o' put next to 'range' is 'ORANGE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for orange that I've seen before include "Princely house" , "Colour between red and yellow" , "Fruit - part of the spectrum" , "stocking filler" , "soft drink?" .)