Cursing terrible grammar (3,8)

I believe the answer is:

bad language

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'cursing' is the definition.
(thesaurus)

'terrible grammar' is the wordplay.
'terrible' becomes 'bad' (similar in meaning).
'grammar' becomes 'language' (I am not sure about this - if you are sure you should believe this answer much more).
'bad'+'language'='BAD LANGUAGE'

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I've seen this clue in The Mirror.

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