Big name in thesauruses (5)
I believe the answer is:
roget
'big name in thesauruses' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roget that I've seen before include "Author of popular English Thesaurus" , "Thesaurus deviser" , "Collector of words" , "Thesaurus publisher" , "English doctor, compiler of a thesaurus, d. 1869" .)