Pouring rain let rot spread (10)

I believe the answer is:

torrential

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'pouring' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'rain let rot spread' is the wordplay.
'spread' indicates an anagram.
'rain'+'let'+'rot'='rainletrot'
'rainletrot' anagrammed gives 'TORRENTIAL'.

Can you help me to learn more?

(Other definitions for torrential that I've seen before include "(Of rain) very heavy" , "Pelting" , "driving" , "Flowing overwhelmingly" , "(Of rain) falling copiously" .)

I've seen this clue in The Times.

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