Write to a chap in prison! (5)

I believe the answer is:

penal

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

'write to a chap in prison' is the definition.
I can't tell whether this defines the answer.

'write to a chap' is the wordplay.
'write' becomes 'pen' (synonyms).
'to a chap' becomes 'al' (man's name).
'pen'+'al'='PENAL'

Can you help me to learn more?

(Other definitions for penal that I've seen before include "Involving legal punishment" , "Relating to legal punishment" , "Punishing" , "Corrective" , "Involving punishment as by the law" .)

I've seen this clue in the Evening Standard.

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