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