Writer of compendium (3)
I believe the answer is:
pen
'writer' is the definition.
(something which writes)
'of compendium' is the wordplay.
'of' says the answer is hidden in the clue.
'PEN' is hidden within 'compendium'.
(Other definitions for pen that I've seen before include "American jug?" , "International writers's organisation" , "Small enclosure" , "See 20" , "record" .)