Additional source of security for customs (5)

I believe the answer is:

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

'additional source of security' is the wordplay.
'additional' becomes 'more' ('more' can be a synonym of 'additional').
'source of' says to take the initial letters.
The first letter of 'security' is 's'.
'more'+'s'='MORES'

'for' acts as a link.

(Other definitions for mores that I've seen before include "Conventions, manners" , "Customs and conventions of a community" , "codes" , "Way of life" , "Fundamental values of a group" .)

I've seen this clue in The Telegraph.

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