Oak apples, for instance, contain the lot (5)

I believe the answer is:

galls

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'oak apples' is the definition.
The definition and answer can be both plants as well as being plural nouns.
Perhaps they are linked in a way I don't understand?

'instance contain the lot' is the wordplay.
'instance' becomes 'gs' (I can't explain this - if you can you should believe this answer much more).
'contain' is an insertion indicator.
'the lot' becomes 'all' (I've seen this before).
'gs' placed around 'all' is 'GALLS'.

'for' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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