Diana could be a nymph (5)
I believe the answer is:
naiad
'a nymph' is the definition.
(I know that naiad is a type of Greek nymph)
'diana could be' is the wordplay.
'could be' is an anagram indicator.
'diana' with letters rearranged gives 'NAIAD'.
(Other definitions for naiad that I've seen before include "Aquatic larva of a dragonfly" , "watery spirit" , "spirit in water" , "Fabulous spring being" , "River nymph" .)