Remove a dunderhead unexpectedly due at court (6)
I believe the answer is:
deduct
'remove' is the definition.
(thesaurus)
'a dunderhead unexpectedly due at court' is the wordplay.
'a dunderhead' becomes 'd' (head letter of 'dunder').
'unexpectedly' is an anagram indicator.
'at' is a charade indicator (letters next to each other).
'court' becomes 'ct'.
'due' with letters rearranged gives 'edu'.
'd'+'edu'+'ct'='DEDUCT'
(Other definitions for deduct that I've seen before include "Remove" , "Knock off" , "Withhold" , "Subtract, take away" , "Make a subtraction" .)