One found inside variety of rose willow (5)
I believe the answer is:
osier
'willow' is the definition.
(I know that osier is a type of willow)
'one found inside variety of rose' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found inside' indicates putting letters inside.
'variety of' indicates an anagram.
'rose' anagrammed gives 'oser'.
'i' placed into 'oser' is 'OSIER'.
(Other definitions for osier that I've seen before include "Willow used in weaving" , "Wetland willow" , "... source of twigs to make baskets" , "Small willow; its shoot" , "Basket material" .)