Part of essay ingeniously used as maxim (6)
I believe the answer is:
saying
'maxim' is the definition.
(thesaurus)
'part of essay ingeniously used' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'SAYING' is hidden within 'essay ingeniously used'.
'as' acts as a link.
(Other definitions for saying that I've seen before include "Epigram" , "Uttering" , "Speaking; adage" , "Articulating" , "Adage, maxim" .)