Idiotic dunderhead back on board (4)
I believe the answer is:
daft
'idiotic' is the definition.
(I've seen this before)
'dunderhead back on board' is the wordplay.
'dunderhead' becomes 'd' (head letter of 'dunder').
'back on board' becomes 'aft' (I've seen this before).
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "Scatty" , "Bats!" , "like a brush?" , "Potty" , "Stupid" .)