Step in front of dog, the leader (10)
I believe the answer is:
pacesetter
'the leader' is the definition.
(I have seen 'temporary leader' mean 'pacesetter' so perhaps 'leader' could also mean 'pacesetter')
'step in front of dog' is the wordplay.
'step' becomes 'pace' ('pace' can be a synonym of 'step').
'in front of' is a charade indicator (letters next to each other).
'dog' becomes 'setter' (setter is a kind of dog**).
'pace'+'setter'='PACESETTER'
(Other definitions for pacesetter that I've seen before include "One determining starting speed of a race" , "early leader in race" , "One who's performance gives the standard for others to reach" , "temporary leader" , "Leading light in one's field, eg fashion, sport, arts" .)