Set aside phone receiver (4-5)
I believe the answer is:
ring-fence
'set aside' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'phone receiver' is the wordplay.
'phone' becomes 'ring' (to ring on the phone).
'receiver' becomes 'fence' (I've seen this before).
'ring'+'fence'='RING-FENCE'
Can you help me to learn more?
(Other definitions for ring-fence that I've seen before include "Protect or guarantee - with circular hedge?" , "Barrier completely enclosing an area of land" , "Encircling barrier" , "Complete barrier" , "Allocate (funds) to one purpose" .)