Spymaster leaves electronic instrument in that place (7)
I believe the answer is:
therein
'in that place' is the definition.
(I've seen this before)
'spymaster leaves electronic instrument' is the wordplay.
'spymaster' becomes 'M' (M is the head of MI6 in James Bond).
'leaves' is a deletion indicator.
'electronic instrument' becomes 'theremin ' (I've seen this before).
'theremin' with 'm' taken away is 'THEREIN'.
(Other definitions for therein that I've seen before include "Neither (anag) - into that" , "enclosed by that place?" , "Inside that" , "in that respect" .)