The Greek goddess of mischief returned her last letter (3)
I believe the answer is:
eta
'letter' is the definition.
(eta is a kind of letter of the alphabet)
'the greek goddess of mischief returned' is the wordplay.
'the greek goddess of mischief' becomes 'ate' (I've seen this before).
'returned' says the letters should be written in reverse.
'ate' written backwards gives 'ETA'.
'her last' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for eta that I've seen before include "Greek letter - Basque nationalist movement" , "giving expected termination of journey" , "European separatists, initially" , "when it might reach us?" , "Member of Greek set a" .)