Without doubt, clock starts in good time (7)
I believe the answer is:
Here is my best explanation:
'without doubt' is the definition.
'clock starts in good time' is the subsidiary indication.
'clock starts' becomes 'cl' (first two letters of clock).
'in good time' becomes 'early' (being early is arriving in good time).
(Other definitions for clearly that I've seen before include "Without doubt or question", "With clarity" and "Obviously; with clarity".)