Stand guard over timer (5)
I believe the answer is:
watch
This might be a double definition.
'stand guard' is the first definition.
(thesaurus)
'over timer' is the second definition.
'watch' can be an answer for 'timer' (thesaurus). I am unsure of the 'over' bit.
Can you help me to learn more?
(Other definitions for watch that I've seen before include "period of duty" , "Keep under observation" , "Timepiece - look out" , "Vigil" , "Look at attentively" .)