Key — it opens throne room from the back (5)
I believe the answer is:
vital
'key' is the definition.
(I've seen this before)
'it opens throne room from the back' is the wordplay.
'opens' indicates putting letters inside (some letters open a gap in others).
'throne room' becomes 'lav' (I can't justify this - if you can you should believe this answer much more).
'from the back' says the letters should be written backwards.
'lav' written backwards gives 'val'.
'it' inserted within 'val' is 'VITAL'.
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Critical, necessary" , "Are such statistics" , "Most important, like certain body organs" , "Critical; life-sustaining" , "Desperately important" .)