What a monk may have's in fashion? Certainly (7)

I believe the answer is:

tonsure

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

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

Can you help me to learn more?

(Other definitions for tonsure that I've seen before include "bare patch" , "Monk's bald spot" , "Monastic haircut" , "Monastic hairstyle" , "Bald bit" .)

I've seen this clue in The Telegraph.

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