Colour on small natural snack (7)
I believe the answer is:
oranges
'natural snack' is the definition.
Although both the answer and 'snack' are nouns, they are not in the same form of the noun.
'colour on small' is the wordplay.
'colour' becomes 'orange' (orange is a kind of colour).
'on small' becomes 's' (abbreviation - e.g. clothes size. I am not sure about the 'on' bit.).
'orange'+'s'='ORANGES'
Can you help me to learn more?
(Other definitions for oranges that I've seen before include "They are not the only fruit, wrote Jeanette Winterson" , "'Mandarins, say (7)'" , "See 8" , "Go snare these fruits" , "Citrus fruits" .)