Diana dressed as 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 dressed' is the wordplay.
'dressed' is an anagram indicator ('dress' can mean 'prepare').
'diana' is an anagram of 'NAIAD'.
'as' is the link.
(Other definitions for naiad that I've seen before include "Nymph of lakes, springs and rivers" , "watery spirit" , "spirit in water" , "Aquatic larva of a dragonfly" , "Mythological water-nymph" .)