Neighbours lose the head with lawsuit with Fiat? (5)

I believe the answer is:

ukase

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'fiat?' is the definition.
(ukase is a kind of fiat)

I cannot understand how the rest of the clue works.

(Other definitions for ukase that I've seen before include "Old Russian edict" , "Peremptory command" , "Official decree, from Russia" , "Imperial decree in Tsarist Russia" , "Tsar's edict" .)

I've seen this clue in the Irish Times.

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