Illogical sailor goes south to old city at the start of December (6)

I believe the answer is:

absurd

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'illogical' is the definition.
(similar in meaning)

'sailor goes south to old city at the start of december' is the wordplay.
'sailor goes' becomes 'ab' (I am not sure about this - if you are sure you should believe this answer much more).
'south' becomes 's' (abbreviation).
'to old city' becomes 'ur' (ancient Sumerian city state).
'at the start of' says to take the initial letters (I've seen 'at the start' mean this).
The initial letter of 'december' is 'd'.
'ab'+'s'+'ur'+'d'='ABSURD'

Can you help me to learn more?

(Other definitions for absurd that I've seen before include "Ridiculous, ludicrous" , "Really ridiculous, silly" , "irrational" , "Preposterous, nonsensical" , "Foolish, quite ridiculous" .)

I've seen this clue in The Irish Times.

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