Dunces love interrupting senior examiner (8)
I believe the answer is:
assessor
'examiner' is the definition.
(I've seen this before)
'dunces love interrupting senior' is the wordplay.
'dunces' becomes 'asses' (ass can mean a dunce or fool).
'love' becomes 'o' (love means zero in tennis).
'interrupting' is an insertion indicator.
'senior' becomes 'sr' (abbreviation).
'o' placed into 'sr' is 'sor'.
'asses'+'sor'='ASSESSOR'
(Other definitions for assessor that I've seen before include "One who estimates value etc for insurance company" , "Person providing marks" , "Official to evaluate property" , "Legal adviser" , "Valuer, estimator" .)