Quick-growing tree with flexible branches (6)
I believe the answer is:
willow
'quick-growing tree with flexible branches' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for willow that I've seen before include "Cricket bat tree" , "It produces osiers" , "Fast-growing tree" , "Pussy- - -" , "Osier; cricket bat" .)