Diana disguised as a nymph (5)
I believe the answer is:
naiad
'a nymph' is the definition.
(naiad is a kind of nymph)
'diana disguised' is the wordplay.
'disguised' indicates an anagram.
'diana' is an anagram of 'NAIAD'.
'as' is the link.
(Other definitions for naiad that I've seen before include "River nymph" , "Aquatic larva of a dragonfly" , "Greek mythological water nymph" , "spirit in water" , "Classic spirit" .)