Familiarity is assumed by the monk (5)

I believe the answer is:

habit

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'familiarity' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for habit that I've seen before include "Acquired pattern of behaviour" , "It's second nature" , "Riding garment" , "Custom; riding dress" , "Breaking this is hard" .)

I've seen this clue in The Irish News.

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