Queen guarded by good fairy (4)
I believe the answer is:
peri
'fairy' is the definition.
(Persian mythological being)
'queen guarded by good' is the wordplay.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'guarded by' indicates putting letters inside.
'good' becomes 'pi' (I've seen this in other clues).
'er' put inside 'pi' is 'PERI'.
(Other definitions for peri that I've seen before include "Fairy (Pers. mythology)" , "Fairy queen" , "Spirit of folklore" , "Malevolent Persian spirit" , "Old Persian fairy" .)