Crossword Genius

Reportedly boy claims hold-alls at post office (8)

Ross

I believe the answer is:

mailbags

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

'reportedly boy claims hold-alls' is the definition.
I can't judge whether this definition defines the answer.

'at post office' is the wordplay.
'at' says to put letters next to each other.
'post' becomes 'mail' (synonyms).
'office' becomes 'bags' (I can't justify this - if you can you should give a lot more credence to this answer).
'mail'+'bags' is 'MAILBAGS'.

Can you help me to learn more?

(Other definitions for mailbags that I've seen before include "Postal sacks" , "Traditionally, they were made in prisons" .)

I've seen this clue in The Irish Times.
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