Remove a dunderhead unexpectedly due at court (6)
I believe the answer is:
deduct
'remove' is the definition.
(I've seen this before)
'a dunderhead unexpectedly due at court' is the wordplay.
'a dunderhead' becomes 'd' (head letter of 'dunder').
'unexpectedly' indicates anagramming the letters.
'at' means one lot of letters go next to another.
'court' becomes 'ct' (abbreviation used in road names).
'due' is an anagram of '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" .)