Flexible tree possibly includes one (6)
I believe the answer is:
Here is my best explanation:
'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 "Malleable, flexible", "docile", "Bendable", "Supple or flexible", "Able to move supply", "Bending easily", "Adaptable", "Tractable", "Flexible - yielding" and "Flexible and easily manipulated".)