Girl who’s wayward so may err (8)
I believe the answer is:
rosemary
'girl' is the definition.
'wayward so may err' is the wordplay.
'wayward' indicates an anagram (wayward means out of control or chaotic).
'so'+'may'+'err'='somayerr'
'somayerr' is an anagram of 'ROSEMARY'.
'who's' is the link.
(Other definitions for rosemary that I've seen before include "Fragrant evergreen shrub and herb" , "such seasons" , "Culinary herb" , "Herb - girl's name(s)" , "Herb with narrow leaves" .)