Aim to swap prison's central characters (4)

I believe the answer is:

goal

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'aim' is the definition.
(I know that aim can be written as goal)

'swap prison's central characters' is the wordplay.
'swap' becomes 'goa' (I can't justify this - if you can you should believe this answer much more).
'prison's' becomes 'cell's' (prison cell).
'central characters' says to take the centre.
The middle letter of 'cells' is 'l'.
'goa'+'l'='GOAL'

'to' is the link.

Can you help me to learn more?

(Other definitions for goal that I've seen before include "Target, objective" , "Objective or score" , "Object, aim" , "Soccer achievement" , "Aim (at football)" .)

I've seen this clue in The Independent.

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