Standard PowerPoint template's first 500 pages (10)

I believe the answer is:

mainstream

'standard' is the definition.
(I know that standard can be written as mainstream)

'powerpoint template's first 500 pages' is the wordplay.
'powerpoint' becomes 'mains' (electrical mains).
'template's first' becomes 't' (1st letter of 'template').
'500 pages' becomes 'ream' (a ream is 500 sheets of paper).
'mains'+'t'+'ream'='MAINSTREAM'

(Other definitions for mainstream that I've seen before include "conventional activities" , "Conventional, normal" , "According to Standard" , "Relating to what is normal or standard" , "entirely conventional?" .)

I've seen this clue in The Independent.

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