Nymph who finally understood (5)
I believe the answer is:
oread
'nymph' is the definition.
(I know that Oread is a type of Greek nymph)
'who finally understood' is the wordplay.
'finally' indicates one should take the final letters.
'understood' becomes 'read' (I've seen this in another clue).
The final letter of 'who' is 'o'.
'o'+'read'='OREAD'
(Other definitions for oread that I've seen before include "classically beautiful girl" , "being in range?" , "Maid of the Mountains" , "Natural spirit" , "O dear, a strange mountain nymph" .)