Fairy came across queen on edge (9)
I believe the answer is:
perimeter
'edge' is the definition.
(perimeter is a kind of edge)
'fairy came across queen on' is the wordplay.
'fairy' becomes 'peri' (Persian mythological being).
'came across' becomes 'met' ('meet' can be a synonym of 'come across').
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'on' means one lot of letters go next to another.
'peri'+'met'+'er'='PERIMETER'
(Other definitions for perimeter that I've seen before include "bounds" , "Border" , "Bound" , "limits" , "edge" .)