Without doubt, clock starts in good time (7)
I believe the answer is:
clearly
'without doubt' is the definition.
(thesaurus)
'clock starts in good time' is the wordplay.
'clock starts' becomes 'cl' (first two letters of clock).
'in good time' becomes 'early' (being early is arriving in good time).
'cl'+'early'='CLEARLY'
(Other definitions for clearly that I've seen before include "Without doubt or question" , "Obviously; with clarity" .)