I came back and consumed waste (8)
I believe the answer is:
emaciate
'waste' is the definition.
(I know that waste can be written as emaciate)
'i came back and consumed' is the wordplay.
'back' is a reversal indicator.
'and' says to put letters next to each other.
'consumed' becomes 'ate' (to eat is to consume food).
'i'+'came'='icame'
'icame' written backwards gives 'emaci'.
'emaci'+'ate'='EMACIATE'
(Other definitions for emaciate that I've seen before include "Waste away" , "Make extremely thin" , "get very thin" .)