Enid's sad end is not exactly known (13)
I believe the answer is:
indeterminate
'not exactly known' is the definition.
(I've seen this before)
'enid's sad end' is the wordplay.
'enid's sad' becomes 'inde' (I can't explain this - if you can you should give a lot more credence to this answer).
'end' becomes 'terminate' (synonyms).
'inde'+'terminate'='INDETERMINATE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for indeterminate that I've seen before include "not exactly fixed" , "Inconclusive" , "Having no fixed value" , "Vague - unsettled" , "Not precisely fixed" .)