Happening all the time, batting stances requiring adjustment (9)
I believe the answer is:
'happening all the time' is the definition.
'batting stances requiring adjustment' is the wordplay.
'batting' becomes 'in' (as in batting or being 'in' in cricket).
'requiring adjustment' indicates anagramming the letters.
'stances' with letters rearranged gives 'cessant'.
(Other definitions for incessant that I've seen before include "Continual and uninterrupted" , "Continuous" , "Constant, unremitting" , "Ceaseless" , "without a break" .)