A couple of girls of fussy disposition (7)
I believe the answer is:
finicky
'of fussy disposition' is the definition.
'a couple of girls' is the wordplay.
'a couple of girls' means two replacements for 'girl'.
'girl' becomes 'fi' (nickname for Fiona).
'girl' becomes 'nicky' (**).
'fi'+'nicky'='FINICKY'
(Other definitions for finicky that I've seen before include "Intricate; fussy" , "particular" .)