Top teachers with sharp competitive advantage (4,5)
I believe the answer is:
head start
'competitive advantage' is the definition.
(I've seen this in another clue)
'top teachers with sharp' is the wordplay.
'top teachers' becomes 'heads' (head is short for headteacher).
'with' means one lot of letters go next to another.
'sharp' becomes 'tart' (I've seen this in another clue).
'heads'+'tart'='HEAD START'
(Other definitions for head start that I've seen before include "'The advantage of beginning ahead of others (4,5)'" , "It improves the chances of success!" , "Boost?" , "An early advantage for the star ad" , "Substantial initial advantage" .)