Two Reds mixed up with one girl (7)
I believe the answer is:
deirdre
'girl' is the definition.
'two reds mixed up with one' is the wordplay.
'two reds' can mean two replacements for 'red'.
'mixed up with' indicates an anagram.
'one' becomes 'i' (Roman numeral).
'red'+'red'='redred'
'redred' anagrammed with 'i' can make 'DEIRDRE'.
(Other definitions for deirdre that I've seen before include "Woman" , "Tragic heroine of Irish myth" , "Female" , "Girl of Irish myth - rid deer (anag)" , "-- of the Sorrows (Synge)" .)