Operations room where available guards are transferred to begin with (7)
I believe the answer is:
theatre
'operations room' is the definition.
(where surgical operations are carried out)
'available guards are transferred to begin with' is the wordplay.
'available' becomes 'there' (I can't justify this - if you can you should give a lot more credence to this answer).
'guards' means one lot of letters goes inside another (inserted letters are guarded).
'to begin with' says to take the initial letters.
The initial letters of 'are transferred' is 'at'.
'there' going around 'at' is 'THEATRE'.
'where' acts as a link.
Can you help me to learn more?
(Other definitions for theatre that I've seen before include "within which one may operate" , "Site of plays or surgery" , "natural land formation?" , "Scene of operations or dramatic performances" , "House" .)