Thin cord (6)
I believe the answer is:
string
'thin cord' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for string that I've seen before include "It's difficult to know how long a piece of it is" , "How long could a piece of it be?" , "Group of horses" , "Collectively, the racehorses trained at one stable" , "Narrow cord or kind of mesh vest" .)