Leaders cutting advantage (4,5)
I believe the answer is:
head start
'advantage' is the definition.
(head start is a kind of advantage)
'leaders cutting' is the wordplay.
'leaders' becomes 'heads' (head is a kind of leader).
'cutting' becomes 'tart' (both can mean sharp or sarcastic).
'heads'+'tart'='HEAD START'
(Other definitions for head start that I've seen before include "An early advantage for the star ad" , "Boost?" , "A good beginning using one's loaf" , "Substantial initial advantage" , "advantage, initially" .)