Brevity of agreement on headland (11)

I believe the answer is:

compactness

'brevity' is the definition.
(thesaurus)

'agreement on headland' is the wordplay.
'agreement' becomes 'compact' (compact can mean an agreement).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'headland' becomes 'ness' (both can mean a promontory).
'compact'+'ness'='COMPACTNESS'

'of' is the link.

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

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