Imagine two girls getting mixed up? (5)
I believe the answer is:
fancy
'imagine' is the definition.
(fancying is a kind of imagining)
I can't explain the remainder of the clue.
(Other definitions for fancy that I've seen before include "Imagine - whim - highly decorated" , "idea" , "Imagine; expensive" , "Imagine; complicated" , "Elaborate; whim" .)