Less than a half share, as a rule? (5)

I believe the answer is:

habit

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'a rule?' is the definition.
(thesaurus)

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

(Other definitions for habit that I've seen before include "It's second nature" , "Monk's gown or custom" , "Propensity" , "Dress - custom - practice" , "Addiction to a drug" .)

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