Superior swaps one for small detector (6)
I believe the answer is:
sensor
'detector' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sensor that I've seen before include "device to detect change?" , "A furtive look" , "Device that reacts to stimulus from snores" , "Detector" , "Detecting device" .)