Extra-singular customs (5)

I believe the answer is:

'customs' is the definition.
(as in social mores)

'extra-singular' is the wordplay.
'extra' becomes 'more' (both can mean additional).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'more'+'s'='MORES'

(Other definitions for mores that I've seen before include "Local customs and conventions" , "Conventions, manners" , "Customs and conventions of society" , "Customs, manners" , "Way of life" .)

I've seen this clue in The Times.

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