Nymph originally named by a retired Welshman (5)
I believe the answer is:
naiad
'nymph' is the definition.
(naiad is a kind of nymph)
'originally named by a retired welshman' is the wordplay.
'originally' says to take the initial letters.
'by' is a charade indicator (letters next to each other).
'retired' says the letters should be written backwards.
'welshman' becomes 'dai' (I've seen this before).
The initial letter of 'named' is 'n'.
'dai' back-to-front is 'iad'.
'n'+'a'+'iad'='NAIAD'
(Other definitions for naiad that I've seen before include "Classic spirit" , "divine female figure" , "Mythological water-nymph" , "spirit in water" , "River nymph" .)