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' means one lot of letters goes inside another.
'senior' becomes 'sr' (abbreviation).
'o' put inside '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" .)