Flexible tree possibly includes one (6)
I believe the answer is:
pliant
'flexible' is the definition.
('pliant' can be a synonym of 'flexible')
'tree possibly includes one' is the wordplay.
'tree possibly' becomes 'plant' (tree is a kind of plant).
'includes' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'plant' placed around 'i' is 'PLIANT'.
(Other definitions for pliant that I've seen before include "Flexible - yielding" , "Supple or flexible" , "easily modified" , "Accommodating" , "Malleable, flexible" .)