Spoils public library? Not on (7)

I believe the answer is:

plunder

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'spoils' is the definition.
(I've seen this in another clue)

'public library? not on' is the wordplay.
'public library?' becomes 'PL' (abbreviation for public library).
'not on' becomes 'under' (I can't justify this - if you can you should give a lot more credence to this answer).
'pl'+'under'='PLUNDER'

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(Other definitions for plunder that I've seen before include "Booty" , "Goods taken" , "Steal goods (in time of war)" , "Ransack" , "Rob" .)

I've seen this clue in The Times.

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