Adult delivery men carry ones defections (10)

I believe the answer is:

apostasies

I'm a little stuck... Click here to teach me more about this clue!

'defections' is the definition.
('apostasy' can be a synonym of 'defection')

'adult delivery men carry ones' is the wordplay.
I cannot quite understand how this works, but
'adult' could be 'a' (abbreviation) and 'a' is present in the answer.
'delivery' could be 'post' (I've seen this before) and 'post' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Guardian.

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