Avidity does good? The reverse! (5)
I believe the answer is:
greed
'avidity' is the definition.
(I've seen this before)
'does good? the reverse' is the wordplay.
'does' becomes 'deer' (I've seen this in other clues).
'good?' becomes 'g' (abbreviation).
'the reverse' says the letters should be written in reverse.
'deer'+'g'='deerg'
'deerg' reversed gives 'GREED'.
(Other definitions for greed that I've seen before include "Voracity" , "Selfish desire" , "Immoderate desire" , "sin" , "Gluttony" .)