Nymph arranged aid, once (7)
I believe the answer is:
oceanid
'nymph' is the definition.
(I know that Oceanid is a type of Greek nymph)
'arranged aid once' is the wordplay.
'arranged' indicates anagramming the letters (letters rearranged).
'aid'+'once'='aidonce'
'aidonce' is an anagram of 'OCEANID'.
(Other definitions for oceanid that I've seen before include "Sea nymph" , "Water nymph, child of Titans" , "Creature of the deep" .)