It's difficult to say how long a piece of it is (6)
I believe the answer is:
string
'it's difficult to say how long a piece of it is' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for string that I've seen before include "This vest is made of large mesh" , "Sequence; cord" , "Length of narrow cord" , "Procession" , "one of four on violin" .)