A grass runs wild, problem growth in the main (9)

I believe the answer is:

sargassum

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'growth in the main' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'a grass runs wild problem' is the wordplay.
'runs wild' indicates an anagram.
'problem' becomes 'sum' (arithmetic problem).
'a'+'grass'='agrass'
'agrass' anagrammed gives 'sargas'.
'sargas'+'sum'='SARGASSUM'

Can you help me to learn more?

(Another definition for sargassum that I've seen is " seaweed".)

I've seen this clue in The Times.

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