Leading by a small margin (5)
I believe the answer is:
ahead
'leading' is the definition.
(synonyms)
'by a small margin' is the wordplay.
'by' means one lot of letters go next to another.
'small margin' becomes 'head' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a' put next to 'head' is 'AHEAD'.
Can you help me to learn more?
(Other definitions for ahead that I've seen before include "In the vanguard" , "Forwards" , "In front as in race" , "advanced" , "at pole" .)