Nothing worrying in a girl's master (8)
I believe the answer is:
original
'master' is the definition.
('original' can be a synonym of 'master')
'nothing worrying in a girl's' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'worrying' indicates anagramming the letters.
'in'+'a'+'girl'='inagirl'
'inagirl' anagrammed gives 'riginal'.
'o'+'riginal'='ORIGINAL'
(Other definitions for original that I've seen before include "In first form" , "Pristine; innovative" , "Archetype,master copy" , "Existing from the start - quite new" , "New and unusual" .)