Gunners receive therapeutic treatment for back passages (5)

I believe the answer is:

recta

'back passages' is the definition.
(plural of rectum)

'gunners receive therapeutic treatment' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'receive' indicates putting letters inside.
'therapeutic treatment' becomes 'ect' (abbreviation for electroconvulsive therapy).
'ra' going around 'ect' is 'RECTA'.

'for' is the link.

I've seen this clue in The Independent.

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