Main cost of the monks (8)

I believe the answer is:

monastic

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'of the monks' is the definition.
The definition suggests a singular noun which matches the answer.

'main cost' is the wordplay.
'main cost' anagrams to 'MONASTIC'.
However, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for monastic that I've seen before include "Withdrawn" , "Brother's" , "Isolated" , "Relating to life in a cloister" , "Of sequestered life under religious vows" .)

I've seen this clue in The Irish News.

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