Prometheus is losing the head, boss (7)

I believe the answer is:

supremo

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'boss' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for supremo that I've seen before include "big Daddy" , "Overall chief" , "he's the tops!" , "Top commander" , "Person invested with great authority" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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