Sleuth's lead (4)
I believe the answer is:
clue
'sleuth's lead' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clue that I've seen before include "Hint to help the solver" , "Luce (anag.)" , "Hint to get what this is?" , "Part of a crossword" , "For example, this" .)