X measures speed (5)
I believe the answer is:
times
This could be a double definition.
'x' is the first definition.
(I've seen this before)
'measures speed' is the second definition.
'times' can be an answer for 'measures' (timing is a kind of measuring). I am unsure of the 'speed' bit.
Can you help me to learn more?
(Other definitions for times that I've seen before include "Big Apple's theatre district" , "operator" , "- - they are a-changin'" , "See 1" , "Paces" .)