The fatheads make only a semi-assessment (5)

I believe the answer is:

asses

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'the fatheads' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for asses that I've seen before include "Small, sure-footed animals" , "Fools, quadrupeds" , "Idiots" , "not the most sensible types" , "Pompous fools" .)

I've seen this clue in the Evening Standard.

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