Author returns to pen good letter in Greek (5)
I believe the answer is:
sigma
'letter in greek' is the definition.
'sigma' can be an answer for 'letter' (letter of the Greek alphabet). I'm unsure of the 'in greek' bit.
'author returns to pen good' is the wordplay.
'author' becomes 'Amis' (Kingsley or Martin Amis).
'returns' shows that the letters should be reversed in order.
'to pen' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'amis' reversed gives 'sima'.
'sima' enclosing 'g' is 'SIGMA'.
Can you help me to learn more?
(Other definitions for sigma that I've seen before include "Agism of the Greek letter" , "Greek letter used as a symbol for summation" , "Greek letter used as summation sign" , "Allowing no relaxation" , "Greek letter signifying sum in maths" .)