Palace guard Newcastle United centre half after eight unbelievable headers (6)
I believe the answer is:
eunuch
'palace guard' is the definition.
(eunuchs were historically used as guards)
I don't understand the remainder of the clue.
(Other definitions for eunuch that I've seen before include "Emasculated man" , "Ineffectual person" , "Powerless type" , "Keeper of a harem" , "Ineffectual man" .)