Crossword Genius

Royal forest fruit? What gall! (3,5)

Ross

I believe the answer is:

oak apple

I'm a little stuck... Click here to teach me more about this clue!

'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!" .)

I've seen this clue in the Evening Standard.
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