Excellent service disrupted by new complaint (4)
I believe the answer is:
acne
'complaint' is the definition.
(acne is a kind of disease)
'excellent service disrupted by new' is the wordplay.
'excellent service' becomes 'ace' (ace is a kind of service. I am not sure about the 'excellent' bit.).
'disrupted by' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'ace' enclosing 'n' is 'ACNE'.
Can you help me to learn more?
(Other definitions for acne that I've seen before include "Spotty coverage" , "Youthful problem" , "Problem to face in" , "Skin complaint" , "Skin problem, mainly of teenagers" .)