Most offensive row is “not English” (7)

I believe the answer is:

rankest

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'most offensive' is the definition.
('rank' can be similar in meaning to 'offensive')

'row is not english' is the wordplay.
'row' becomes 'rank' (as in a taxi rank).
'is not english' becomes 'est' (I can't justify this - if you can you should give a lot more credence to this answer).
'rank'+'est'='RANKEST'

Can you help me to learn more?

(Other definitions for rankest that I've seen before include "most objectionable" , "most offensive" .)

I've seen this clue in The Times.

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