Idiot favouring rubbish principle of The Avengers? (3,3,3)
I believe the answer is:
tit for tat
'principle of the avengers?' is the definition.
(you might avenge someone as tit for tat)
'idiot favouring rubbish' is the wordplay.
'idiot' becomes 'tit' (both can mean a stupid person).
'favouring' becomes 'for' (to be 'for' something is to be in favour of it).
'rubbish' becomes 'tat' (low-quality items or material).
'tit'+'for'+'tat'='TIT-FOR-TAT'
(Other definitions for tit for tat that I've seen before include "You hit me, I hit you" , "Retaliation in kind" , "Repayment in kind" , "satisfaction" , "An eye for an eye" .)