Secretion from plants that could be resuming (3,5)

I believe the answer is:

gum resin

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

'secretion from plants' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'could be resuming' is the wordplay.
'could be' is an anagram indicator.
'resuming' anagrammed gives 'GUM RESIN'.

'that' acts as a link.

Can you help me to learn more?

(Another definition for gum resin that I've seen is " gamboge...".)

I've seen this clue in The Independent.

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