Endless mysterious instances (9)
I believe the answer is:
incessant
'endless' is the definition.
(I've seen this before)
'mysterious instances' is the wordplay.
'mysterious' is an anagram indicator.
'instances' is an anagram of 'INCESSANT'.
(Other definitions for incessant that I've seen before include "Continual and uninterrupted" , "Constant, unremitting" , "Nine casts (anag) - nonstop" , "Unending" , "happening all the time" .)