Constant pain regularly found in flying insects (9)
'constant' is the definition.
'pain regularly found in flying insects' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'found in' is an insertion indicator.
'flying' indicates anagramming the letters.
The alternating letters of 'pain' are 'an'.
'insects' is an anagram of 'incesst'.
'an' put within 'incesst' is 'INCESSANT'.
(Other definitions for incessant that I've seen before include "Nine casts (anag) - nonstop", "Continual and uninterrupted", "Unending", "Instances (anag)", "Perpetual, unremitting".)