Nymph: Diana, possibly (5)
I believe the answer is:
naiad
'nymph' is the definition.
(naiad is a kind of nymph)
'diana possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'diana' with letters rearranged gives 'NAIAD'.
(Other definitions for naiad that I've seen before include "spirit in water" , "Fabulous spring being" , "Aquatic larva of a dragonfly" , "watery spirit" , "River nymph" .)