Almost rate as dunderheads (5)
I believe the answer is:
asses
'dunderheads' is the definition.
(I've seen this in another clue)
'almost rate' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'rate' becomes 'assess' (I've seen this before).
'assess' with its final letter removed is 'ASSES'.
'as' acts as a link.
(Other definitions for asses that I've seen before include "Pompous fools" , "dim bulbs?" , "Small, sure-footed animals" , "Idiots" , "Fools; beasts of burden" .)