Mended by a unique dunderhead (5)
I believe the answer is:
soled
'mended' is the definition.
(soling is a kind of mending)
'by a unique dunderhead' is the wordplay.
'by' says to put letters next to each other.
'a unique' becomes 'sole' (I've seen this in another clue).
'dunderhead' becomes 'd' (head letter of 'dunder').
'sole' next to 'd' is 'SOLED'.
(Other definitions for soled that I've seen before include "Repaired shoe" , "Pulled at one time" .)