Double agents don't get stoned in the expensive fish restaurant (6)
I believe the answer is:
beluga
'fish restaurant' is the definition.
'beluga' can be an answer for 'fish' (I've seen this before). I'm unsure of the 'restaurant' bit.
'double agents don't get stoned in the expensive' is the wordplay.
I cannot quite see how this works, but
'get' could be 'be' ('get' can mean 'come to be') and 'be' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for beluga that I've seen before include "Great Russian sturgeon - Arctic whale" , "Large sturgeon or it's caviare" , "fish" , "'Russian sturgeon, source of caviar (6)'" , "Small white whale; type of caviar" .)