One thus about to tour Italian port dealing with marks on a map (8)

I believe the answer is:

isobaric

I'm a little stuck... Click here to teach me more about this clue!

'dealing with marks on a map' is the definition.
I can't tell whether this definition defines the answer.

'one thus about to tour italian port' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thus' becomes 'so' (synonyms).
'about' becomes 'c' (circa).
'to tour' means one lot of letters goes inside another (to tour can mean to go around).
'italian port' becomes 'bari' (I've seen this in another clue).
'so'+'c'='soc'
'soc' enclosing 'bari' is 'sobaric'.
'i'+'sobaric'='ISOBARIC'

Can you help me to learn more?

(Another definition for isobaric that I've seen is " of atmospheric contours".)

I've seen this clue in The Guardian.

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