Changing to stingier rates gets complaint (15)
I believe the answer is:
gastroenteritis
'complaint' is the definition.
(gastroenteritis is a kind of disease)
'changing to stingier rates' is the wordplay.
'changing' is an anagram indicator.
'to'+'stingier'+'rates'='tostingierrates'
'tostingierrates' with letters rearranged gives 'GASTROENTERITIS'.
'gets' is the link.
(Other definitions for gastroenteritis that I've seen before include "Corporation complaint" , "Inflammation of the stomach and intestines" .)