The willowy plant contains one (6)
I believe the answer is:
pliant
'the willowy' is the definition.
(thesaurus)
'plant contains one' is the wordplay.
'contains' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'plant' enclosing 'i' is 'PLIANT'.
(Other definitions for pliant that I've seen before include "Tractable" , "Plastic" , "Adaptable" , "easily controlled" , "Malleable, flexible" .)