Step ahead of dog, one temporarily leading (4-6)
I believe the answer is:
pace setter
'one temporarily leading' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'step ahead of dog' is the wordplay.
'step' becomes 'pace' (pace is a kind of step).
'ahead of' says to put letters next to each other (some letters go in front of others).
'dog' becomes 'setter' (setter is a type of dog).
'pace'+'setter'='PACE-SETTER'
Can you help me to learn more?
(Other definitions for pace setter that I've seen before include "race's early leader" , "One has lead" , "one foremost in field?" , "we want a leader here" , "He leads" .)