It usually changes 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 usually changes from green to' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'to' is within the answer.
The remaining letters 'ma' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tomato that I've seen before include "that's suitable to eat" , "A glossy red fruit eaten as a vegetable" , "It used to be call a love-apple" , ""Plum" or "beef" fruit?" , "It's also known as a love apple" .)