Greek character and relations — about 100 — work together (4,2)
I believe the answer is:
muck in
'greek character and relations about 100 work together' is the definition.
I can't judge whether this defines the answer.
'greek character and relations about 100' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'and' is a charade indicator (letters next to each other).
'relations' becomes 'kin' (both can mean one's family).
'about' says to put letters next to each other.
'100' becomes 'C' (Roman numeral).
'kin' after 'c' is 'ckin'.
'mu'+'ckin' is 'MUCK IN'.
Can you help me to learn more?
(Other definitions for muck in that I've seen before include "Do one's bit" , "Help out" , "Lend a hand" , "In slang, share tasks equally - even filthy ones" .)
