Crossword Genius

When reported, mother must call on old prison (10)


I believe the answer is:


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

'when reported mother must call' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'mother' could be 'ma' (Ma is another word for mother or mum) and 'ma' is found in the answer.
This explanation may well be incorrect...

'on' is the link.

Can you help me to learn more?

(Other definitions for marshalsea that I've seen before include "Old prison" , "Former London debtors' prison" , "It kept prisoners" .)

I've seen this clue in The Independent.
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