Need about one twentieth of a ream of paper (7)

I believe the answer is:

require

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'need' is the definition.
(I know that need can be written as require)

'about one twentieth of a ream of paper' is the wordplay.
'about' shows that the letters should be reversed in order.
'one twentieth of a ream of' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'paper' becomes 'quire' (unit for an amount of paper).
'er' written backwards gives 're'.
're'+'quire'='REQUIRE'

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(Other definitions for require that I've seen before include "Insist on" , ".. must have" , "Want, demand" , "exact" , "26 [ORDER]" .)

I've seen this clue in The Times.

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