Leading sheep of a flock (10)
I believe the answer is:
bellwether
'leading sheep of a flock' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bellwether that I've seen before include "Leading sheep of a flock" , "Leader of the flock" , "Sheep that leads the flock -- we tell Herb (anag)" , "Leading sheep; trendsetter" .)