Crossword Genius

Rival’s plan spreading fast (5)

Ross

I believe the answer is:

viral

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

'spreading fast' is the definition.
Although both the answer and definition are adjectives, I cannot understand how they can define each other.

'rival's plan' is the wordplay.
'plan' indicates anagramming the letters (plan can mean organise).
'rival' is an anagram of 'VIRAL'.

Can you help me to learn more?

(Other definitions for viral that I've seen before include "Like many diseases" , "Like some infections" , "Like pathological agents" , "as an internet sensation" , "Spreading rapidly" .)

I've seen this clue in The Mirror.
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