Being in jail, in custody receiving constant allowance (13)

I believe the answer is:

incarceration

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'being in jail' is the definition.
'incarceration' can be an answer for 'jail' (I've seen this before). I am not sure about the 'being in' bit.

'in custody receiving constant allowance' is the wordplay.
'custody' becomes 'care' (I've seen this before).
'receiving' indicates putting letters inside.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'allowance' becomes 'ration' (I've seen this before).
'care' placed around 'c' is 'carce'.
'in'+'carce'+'ration'='INCARCERATION'

Can you help me to learn more?

(Other definitions for incarceration that I've seen before include "Jail" , "Judicial confinement" , "Imprisonment" , "Captivity" .)

I've seen this clue in The Times.

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