English writer upset about first letter from Greece — or a later one (5)
I believe the answer is:
sigma
'a later one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'english writer upset about first letter from greece' is the wordplay.
'english writer' becomes 'amis' (either Kingsley Amis or Martin Amis).
'upset' shows that the letters should be reversed in order.
'about' is an insertion indicator.
'first letter from greece' becomes 'g' (1st letter of 'greece').
'amis' written backwards gives 'sima'.
'sima' going around 'g' is 'SIGMA'.
'or' acts as a link.
(Other definitions for sigma that I've seen before include "letter to Greeks" , "Greek letter used as summation sign" , "Agism of the Greek letter" , "Greek letter corresponding to S" , "Greek letter indicating summation in maths" .)