Greek character fiddled with pen describing storage site (7)
I believe the answer is:
epsilon
'greek character' is the definition.
(letter from the Greek alphabet)
'fiddled with pen describing storage site' is the wordplay.
'fiddled with' indicates an anagram.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'storage site' becomes 'silo' (I've seen this in another clue).
'pen' anagrammed gives 'epn'.
'epn' placed around 'silo' is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "foreign letter" , "5th letter of the Greek alphabet" , "Greek character" , "Greek short E" , "Fifth brightest star in a constellation" .)