Volume of customers (4)

I believe the answer is:

tome

'volume' is the definition.
(I've seen this before)

'of customers' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TOME' can be found hidden inside 'customers'.

(Other definitions for tome that I've seen before include "War and Peace, say" , "Large heavy book" , "dictionary?" , "A hefty book" , "Big book" .)

I've seen this clue in The Guardian.

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