Greek character left surrounded by wild ponies (7)
I believe the answer is:
epsilon
'greek character' is the definition.
(letter from the Greek alphabet)
'left surrounded by wild ponies' is the wordplay.
'left' becomes 'l' (common abbreviation).
'surrounded by' means one lot of letters goes inside another.
'wild' is an anagram indicator.
'ponies' is an anagram of 'epsion'.
'l' put within 'epsion' is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "Foreign character" , "character in Greece" , "Fifth brightest star in a constellation" , "5th letter of the Greek alphabet" , "Fifth letter of the Greek alphabet" .)