Nymphs concealed in reeds (7)
I believe the answer is:
nereids
'nymphs' is the definition.
(I've seen this before)
'concealed in reeds' is the wordplay.
'concealed' is an anagram indicator.
'in'+'reeds'='inreeds'
'inreeds' with letters rearranged gives 'NEREIDS'.
(Other definitions for nereids that I've seen before include "Group of nymphs" , "Need, sir, could produce seanymphs" , "Mythological sea nymphs" .)