It suggests promise of purpose is/is not unusual (14)

I believe the answer is:

propitiousness

'it suggests promise' is the definition.
(propitiousness is the suggestion of success)

'purpose isis not unusual' is the wordplay.
'unusual' indicates anagramming the letters.
'purpose'+'isis'+'not'='purposeisisnot'
'purposeisisnot' is an anagram of 'PROPITIOUSNESS'.

'of' is the link.

I've seen this clue in The Independent.

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