Leader seeing champ's behind in guaranteed second (7)
I believe the answer is:
supremo
'leader' is the definition.
(I've seen this before)
'champ's behind in guaranteed second' is the wordplay.
'behind' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'guaranteed' becomes 'sure' (I've seen this in another clue).
'second' becomes 'mo' (both are short periods of time).
The final letter of 'champ' is 'p'.
'p' going inside 'sure' is 'supre'.
'supre'+'mo'='SUPREMO'
'seeing' acts as a link.
(Other definitions for supremo that I've seen before include "One in overall charge" , "Leader with unlimited powers" , "Overall chief" , "Person in overall authority" , "Big cheese, head honcho" .)