Up or down detector (5)
I believe the answer is:
radar
This clue is a double definition.
'up or down' is the first definition.
'detector' is the second definition.
(I've seen this before)
(Other definitions for radar that I've seen before include "Palindromic tracking system" , "Range and direction finding equipment" , "Technique using beam" , "Radio detection system" , "Aircraft detection system" .)