But, to a girl, he's Romeo! (6)
I believe the answer is:
oliver
'he's romeo' is the definition.
The definition suggests a singular noun but the answer is not.
'but to a girl' is the wordplay.
'but' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'to a' says to put letters next to each other (I've seen this in other clues).
'girl' becomes 'Olive' (girl's name).
'r' put after 'olive' is 'OLIVER'.
Can you help me to learn more?
(Other definitions for oliver that I've seen before include "See 19" , "Dickens-based musical" , "Dickensian musical" , "One taking pride in food" , "- Goldsmith (She Stoops to Conquer)" .)