Greek character, he leaves home with one councillor ... no return (7)
I believe the answer is:
omicron
'greek character' is the definition.
(letter from the Greek alphabet)
'he leaves home with one councillor no return' is the wordplay.
'leaves' suggests deleting specific letters.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'councillor' becomes 'cr' (abbreviation for councillor).
'return' says the letters should be written in reverse.
'home' with 'he' taken away is 'om'.
'no' in reverse letter order is 'on'.
'om'+'i'+'cr'+'on'='OMICRON'
(Other definitions for omicron that I've seen before include "Moronic (anag) - Greek letter" , "Greek character" , "old letter" , "Foreign letter -- I'm no roc (anag)" , "Round character" .)