She was one of his heroines (5,7)
I believe the answer is:
rider haggard
'his heroines' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'she was one' is the wordplay.
I cannot really understand how this works, but
'she' could be 'de' and 'de' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for rider haggard that I've seen before include "English novelist" , "Writer" , "King Solomon's Mines author" .)