Leading a bike race - chortle - by 50 minutes! (2,3,4)
I believe the answer is:
at the helm
'leading' is the definition.
(to be at the helm is to lead or be in charge)
'a bike race chortle by 50 minutes' is the wordplay.
'bike race' becomes 'tt' (short for time trial, type of race).
'chortle' becomes 'hehe' (a short laugh).
'by' is a charade indicator (letters next to each other).
'50' becomes 'l' (roman numeral).
'minutes' becomes 'm' (common abbreviation).
'a'+'tt'+'hehe'+'l'+'m'='AT THE HELM'
(Another definition for at the helm that I've seen is " Directing affairs".)