A Mass for Gareth Cook (6)
I believe the answer is:
gather
'a mass' is the definition.
(thesaurus)
'gareth cook' is the wordplay.
'cook' indicates anagramming the letters.
'gareth' is an anagram of 'GATHER'.
'for' is the link.
(Other definitions for gather that I've seen before include "Assemble - conclude - contract" , "put in pleats" , "Come together for the rag" , "Get together" , "Accumulate, amass" .)