Unusual for vipers to be so frolicsome (8)

I believe the answer is:

sportive

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'frolicsome' is the definition.
(I know that frolicsome can be written as sportive)

'unusual for vipers to be' is the wordplay.
'vipers to' anagrams to 'SPORTIVE'.
However, I'm unsure how this is indicated.

'so' is the link.

Can you help me to learn more?

(Other definitions for sportive that I've seen before include "frisky" , "merry" .)

I've seen this clue in The Globe and Mail.

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