In harmony, lacking a yen to be avaricious (6)
I believe the answer is:
greedy
'be avaricious' is the definition.
('greedy' can be a synonym of 'avaricious')
'in harmony lacking a yen' is the wordplay.
'in harmony' becomes 'agreed' (I've seen this before).
'lacking' suggests deleting specific letters.
'yen' becomes 'y'.
'agreed' with 'a' removed is 'greed'.
'greed'+'y'='GREEDY'
'to' acts as a link.
(Other definitions for greedy that I've seen before include "Starving" , "pig!" , "Overeating" , "Keen" , "Covetous, avaricious" .)