Diana madly in the swim (5)
I believe the answer is:
naiad
'the swim' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'diana madly' is the wordplay.
'madly' is an anagram indicator.
'diana' is an anagram of 'NAIAD'.
'in' is the link.
Can you help me to learn more?
(Other definitions for naiad that I've seen before include "watery spirit" , "Aquatic larva of a dragonfly" , "Greek mythological water nymph" , "Nymph of lakes, springs and rivers" , "Diana (anag.)" .)