Travelling theatre is most vulgar (9)

I believe the answer is:

earthiest

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'most vulgar' is the definition.
Both the definition and answer are superlative adjectives. Maybe there's an association between them I don't understand?

'travelling theatre is' is the wordplay.
'travelling' indicates anagramming the letters.
'theatre'+'is'='theatreis'
'theatreis' is an anagram of 'EARTHIEST'.

Can you help me to learn more?

(Another definition for earthiest that I've seen is " The most coarse".)

I've seen this clue in The Telegraph.

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