Royal forest fruit? What gall! (3,5)
I believe the answer is:
oak apple
'what gall' is the definition.
(I've seen this before)
'royal forest fruit?' is the wordplay.
'royal forest' becomes 'oak' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fruit?' becomes 'apple' (apple is a kind of fruit).
'oak'+'apple'='OAK-APPLE'
Can you help me to learn more?
(Other definitions for oak apple that I've seen before include "Gall on tree - day, 29 May" , "Gall on some trees" , "Abnormal growth" , "Charles II's day?" , "restoring the monarchy on this day!" .)