Couples at front in right tunic allowable for ceremony (6)

I believe the answer is:

ritual

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'for ceremony' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for ritual that I've seen before include "Customary (eg religious) practice" , "Stereotyped behaviour" , "Codified ceremony" , "Regular habit" , "Ceremony or service" .)

I've seen this clue in The Times.

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