Water nymphs in a sad muddle (6)
I believe the answer is:
naiads
'water nymphs' is the definition.
(nymphs in Greek myth)
'in a sad muddle' is the wordplay.
'muddle' indicates an anagram.
'in'+'a'+'sad'='inasad'
'inasad' with letters rearranged gives 'NAIADS'.
(Other definitions for naiads that I've seen before include "Legendary swimmers" , "Water nymphs - sad Ian (anag)" , "Greek water nymphs" , "Water-nymphs" , "Mythological water nymphs" .)