It's matured and gone off? Initially, yes (5)
I believe the answer is:
imago
'yes' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it's matured and gone off? initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'its matured and gone off' is 'IMAGO'.
Can you help me to learn more?
(Other definitions for imago that I've seen before include "Sexually mature adult insect" , "idealised figure" , "subconscious view" , "Last stage of insect's development" , "spinner, say" .)