Rosie ran round tree (5)
I believe the answer is:
osier
'tree' is the definition.
(osier is a kind of tree)
'rosie ran round' is the wordplay.
'ran round' is an anagram indicator (I've seen 'running round' mean this).
'rosie' anagrammed gives 'OSIER'.
(Other definitions for osier that I've seen before include "Wetland willow" , "Basket-maker's willow" , "Willow for basket-making" , "Tree - Rosie (anag)" , "Small willow; its shoot" .)