Certain advert entertained a portion of them but it was unintentional (11)

I believe the answer is:

inadvertent

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'unintentional' is the definition.
(I've seen this in another clue)

'certain advert entertained a portion of' is the wordplay.
'a portion of' says the answer is hidden in the clue.
'INADVERTENT' is hidden in the letters of 'certain advert entertained'.

'them but it was' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for inadvertent that I've seen before include "unguarded" , "Accidental" , "Careless" , "Unintentional" , "without meaning" .)

I've seen this clue in The Irish Times.

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