Use a stopwatch on (4)
I believe the answer is:
time
'use a stopwatch on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for time that I've seen before include "'Every ... we say goodbye, I cry a little' /Cole Porter" , "'. . . and tide wait for no man'" , "Spell" , "Rhythm" , "weekly magazine" .)