Princess, endlessly beloved by yours truly (5)
I believe the answer is:
medea
'princess' is the definition.
(I have seen 'fabulous princess' mean 'medea' so perhaps 'princess' could also mean 'medea')
'endlessly beloved by yours truly' is the wordplay.
'endlessly beloved' becomes 'dea' (I can't justify this - if you can you should believe this answer much more).
'by' means one lot of letters go next to another.
'yours truly' becomes 'me' (the person who set this crossword).
'dea' put after 'me' is 'MEDEA'.
Can you help me to learn more?
(Other definitions for medea that I've seen before include "Mythological Greek princess" , "Mythical wife who helped Jason get the Golden Fleece" , "Greek princess who helped Jason obtain the Golden Fleece" , "fabulous princess" , "Tragedy by Euripedes" .)