One needs to set out to be flexible (6)
I believe the answer is:
pliant
'flexible' is the definition.
(I know that flexible can be written as pliant)
'one needs to set out' is the wordplay.
'one needs' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'to set' becomes 'plant' (planting is a kind of setting).
'out' means one lot of letters goes inside another.
'i' placed within 'plant' is 'PLIANT'.
'to be' is the link.
Can you help me to learn more?
(Other definitions for pliant that I've seen before include "Flexible, easy to bend" , "easily controlled" , "Flexible and easily manipulated" , "Not firm" , "Tractable" .)