Writer of compendium (3)
I believe the answer is:
pen
'writer' is the definition.
(thesaurus)
'of compendium' is the wordplay.
'of' indicates the answer is hidden within 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" .)