Flabbergasted at cagoule in mail-order book (9)

I believe the answer is:

catalogue

I'm a little stuck... Click here to teach me more about this clue!

'book' is the definition.
(catalogue is a kind of book)

'flabbergasted at cagoule in mail-order' is the wordplay.
'at cagoule' anagrams to 'CATALOGUE'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for catalogue that I've seen before include "Brochure containing a list of goods for sale" , "Systematic list of items" , "(Ilustrated) list of items" , "List of items for sale" , "Actual ego involved in list of items or goods" .)

I've seen this clue in The Telegraph.

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