"The Little Mermaid" villain (6)
I believe the answer is:
ursula
'the little mermaid villain' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ursula that I've seen before include "Legendary saint said to be martyred with virgins" , "Martyred female saint" , "- - Andress, the original Bond girl" , "Woman" .)