Chat with rest in Ritz regularly about complaint (9)
I believe the answer is:
gastritis
'complaint' is the definition.
(gastritis is a kind of disease)
'chat with rest in ritz regularly about' is the wordplay.
'chat' becomes 'gas' (informal term meaning to chat).
'with' is a charade indicator (letters next to each other).
'rest' becomes 'sit' (I've seen this before).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'about' shows that the letters should be reversed in order.
'in'+'ritz'='inritz'
The alternate letters of 'inritz' are 'irt'.
'sit'+'irt'='sitirt'
'sitirt' back-to-front is 'tritis'.
'gas'+'tritis'='GASTRITIS'
(Other definitions for gastritis that I've seen before include "medical problem" , "Stomach complaint" , "stomach problem" , "stomach upset" , "Stomach inflammation" .)