A lot of enthusiasm after receiving tense letter from Athens (4)
I believe the answer is:
zeta
'letter from athens' is the definition.
'a lot of enthusiasm after receiving tense' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'enthusiasm' becomes 'zeal' (zeal is a kind of enthusiasm).
'after receiving' is an insertion indicator.
'tense' becomes 't' (abbreviation used in many dictionaries).
'zeal' with its final letter removed is 'zea'.
'zea' going around 't' is 'ZETA'.
(Other definitions for zeta that I've seen before include "6th Greek letter" , "Greek letter (for Catherine Jones?)" , "Welsh actress, Catherine __-Jones, b. 1969" , "Letter to the Corinthians" , "Spartan character" .)