Release compendium at no cost (3,4)

I believe the answer is:

set free

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'release' is the definition.
(synonyms)

'compendium at no cost' is the wordplay.
'compendium at' becomes 'set' (I am not sure about this - if you are sure you should believe this answer much more).
'no cost' becomes 'free' (I've seen this in other clues).
'set'+'free'='SET FREE'

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(Other definitions for set free that I've seen before include "without the box" , "Deliver - release" , "Liberate" , "Released" , "no longer detained" .)

I've seen this clue in The Sun.

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