Frequently expressed as decimal (5)

I believe the answer is:

often

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'frequently' is the definition.
(synonyms)

I don't understand how the remainder of the clue works.

(Other definitions for often that I've seen before include "Much of the time" , "on many occasions" , "Repeatedly with regularity" , "occurring with frequency" , "Commonly" .)

I've seen this clue in The Globe and Mail.

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