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