Guard, to some extent, present in Elsinore (8)
I believe the answer is:
sentinel
'guard' is the definition.
(I know that sentinel is a type of security guard)
'to some extent present in elsinore' is the wordplay.
'to some extent' says the answer is hidden in the clue (I've seen 'extent of' mean this (I've seen this in other clues)).
'SENTINEL' is hidden in the letters of 'present in elsinore'.
(Other definitions for sentinel that I've seen before include "Watchman" , "Soldier keeping watch" , "One watches" , "One watching" , "Guard - ten lines (anag)" .)