Ultimately plaintive PS in long, detailed letter from Athens (7)
I believe the answer is:
epsilon
'letter from athens' is the definition.
(letter of the Greek alphabet)
'ultimately plaintive ps in long detailed' is the wordplay.
'ultimately' says to take the final letters.
'ps in' becomes 'psi' (I can't explain this - if you can you should believe this answer much more).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
The last letter of 'plaintive' is 'e'.
'long' with its final letter taken off is 'lon'.
'e'+'psi'+'lon'='EPSILON'
Can you help me to learn more?
(Other definitions for epsilon that I've seen before include "character in Greece" , "Fifth brightest star in a constellation" , "Lose pin (anag.)" , "Foreign character" , "small number mispronounced, almost completely the wrong way round" .)