One of 78 in Buckingham Palace (3)
I believe the answer is:
loo
'one of 78 in buckingham palace' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for loo that I've seen before include "Toilet (informal)" , "In which one goes" , "bog" , "A colloquial toilet" , "Privy" .)