Constant undoing regularly upset kitties (7)
I believe the answer is:
nonstop
'constant' is the definition.
(I've seen this before)
'undoing regularly upset kitties' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'upset' shows that the letters should be reversed in order.
'kitties' becomes 'pots' ('pot' can be a synonym of 'kitty').
The alternating letters of 'undoing' are 'non'.
'pots' reversed gives 'stop'.
'non'+'stop'='NONSTOP'
(Other definitions for nonstop that I've seen before include "Done without pause" , "without rest" , "Express" , "without a break" , "done without interruption" .)