Mythological water nymphs (6)
I believe the answer is:
naiads
'mythological water nymphs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for naiads that I've seen before include "Female swimmers" , "Water nymphs - sad Ian (anag)" , "Mythological water nymphs" , "Classical water nymphs" , "Legendary swimmers" .)