Impulse to place in box part of transmission (4,4)
I believe the answer is:
spur gear
'box part of transmission' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'impulse to place' is the wordplay.
'impulse' becomes 'spur' (I've seen this before).
'to place' becomes 'gear' (I can't justify this - if you can you should give a lot more credence to this answer).
'spur'+'gear'='SPUR-GEAR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for spur gear that I've seen before include "Toothed wheel" , "used to transmit power" .)