Exact ending is missing from the summary (6)
I believe the answer is:
precis
'the summary' is the definition.
(precis is a kind of summary)
'exact ending is missing' is the wordplay.
I cannot really see how this works, but
'is' is within the answer.
This explanation may well be incorrect...
'from' acts as a link.
Can you help me to learn more?
(Other definitions for precis that I've seen before include "Abstract; make summary (of text)" , "Prices for a summary" , "Synopsis, summary" , "Prices for abbreviated version" , "Cripes, what a summary!" .)