Kite-flying need (6)
I believe the answer is:
'kite-flying need' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for string that I've seen before include "Procession" , "Thin piece of cord" , "Collectively, the racehorses trained at one stable" , "might one be plucked?" , "Succession" .)