Croatian city quartet in limitless grammatical error (5,10)

I believe the answer is:

split infinitive

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'grammatical error' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

'croatian city quartet in limitless' is the wordplay.
'croatian city' becomes 'split' (I have seen 'European city' mean 'split' so perhaps 'city' could also mean 'split'. I am not sure about the 'croatian' bit.).
'quartet' becomes 'IV' (Roman numerals for four).
'in' is an insertion indicator.
'limitless' becomes 'infinite' (similar in meaning).
'iv' inserted into 'infinite' is 'infinitive'.
'split'+'infinitive'='SPLIT INFINITIVE'

Can you help me to learn more?

(Other definitions for split infinitive that I've seen before include "To boldly go" , "structure some object to" , "Grammatical error to many" .)

I've seen this clue in the Sydney Morning Herald.

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