Writers of a footnote about certain points (4)

I believe the answer is:

pens

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'writers' is the definition.
(I've seen this before)

'a footnote about certain points' is the wordplay.
'a footnote' becomes 'ps'.
'about' indicates putting letters inside.
'certain points' becomes 'en' (I am not sure about the 'certain' bit.**).
'ps' enclosing 'en' is 'PENS'.

'of' is the link.

Can you help me to learn more?

(Other definitions for pens that I've seen before include "Literary thought or reflection" , "Imprisons" , "Writing implements" , "Dashes off" , "birds" .)

I've seen this clue in the Evening Standard.

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