It turns from green to red (6)
I believe the answer is:
tomato
'red' is the definition.
(I know that tomato is a type of red)
'it turns from green to' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'turns from green' becomes 'oma' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'oma'+'to'='TOMATO'
Can you help me to learn more?
(Other definitions for tomato that I've seen before include "Also called the love apple" , "In France, it's a love apple" , "Pulpy fruit eaten as vegetable" , "Type of sauce" , "It used to be call a love-apple" .)