Persistent customers have registered over ninety individual complaints to start with (7)
I believe the answer is:
chronic
'persistent' is the definition.
(a chronic condition is a persistent one)
'customers have registered over ninety individual complaints to start with' is the wordplay.
'to start with' says to take the initial letters.
The first letters of 'customers have registered over ninety individual complaints' is 'CHRONIC'.
(Other definitions for chronic that I've seen before include "Long-continuing" , "(Of disease) persistent" , "Very serious" , "(Of a problem) long-lasting" , "(Of illness) slow developing over a long time" .)