Keep an eye on the clock (5)
I believe the answer is:
watch
I believe this clue is a double definition.
'keep an eye on' is the first definition.
(synonyms)
'clock' is the second definition.
(I know that watch is a type of clock)
'the' acts as a link.
(Other definitions for watch that I've seen before include "Timepiece - look out" , "Keep under observation" , "Look at attentively" , "Keep an eye on - timepiece" , "See 21" .)