Two things eyewitness does with a timer (9)
I believe the answer is:
stopwatch
'timer' is the definition.
(I know that stopwatch is a type of timer)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stopwatch that I've seen before include "Timepiece used in racing" , "Timing device, for sports perhaps" , "instrument" , "Timer for races, etc" , "Timepiece for recording speed of racers" .)