A retest? I revised for essay (8)
I believe the answer is:
treatise
'essay' is the definition.
(I've seen this before)
'a retest? i revised' is the wordplay.
'revised' indicates an anagram.
'a'+'retest'+'i'='aretesti'
'aretesti' anagrammed gives 'TREATISE'.
'for' is the link.
(Other definitions for treatise that I've seen before include "Learned paper" , "Academic composition" , "Tract, monograph" , "Iterates in dissertation" , "Written exposition of an argument" .)