Sometimes undergoing metamorphosis, as in cocoon (2,8)

I believe the answer is:

on occasion

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'sometimes' is the definition.
(I've seen this in another clue)

'metamorphosis as in cocoon' is the wordplay.
'metamorphosis' indicates an anagram.
'as'+'in'+'cocoon'='asincocoon'
'asincocoon' with letters rearranged gives 'ON OCCASION'.

'undergoing' is the link.
I am not very happy about this link. It may be part of another bit of the clue.

Can you help me to learn more?

(Another definition for on occasion that I've seen is " Sometimes".)

I've seen this clue in The Everyman puzzle in The Observer.

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