Not a fit face for an assumption of pretence (11)

I believe the answer is:

affectation

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'pretence' is the definition.
(affectation is a kind of pretence)

'not a fit face for an assumption' is the wordplay.
'not a fit face' can be anagrammed to 'AFFECTATION'.
But, I'm unsure how the anagram is indicated.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for affectation that I've seen before include "unnatural appearance" , "retence" , "Show evidence of" , "Deliberate pretence" .)

I've seen this clue in The Irish News.

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