Stuff provided sent back before lunch? (6)
I believe the answer is:
defeat
'stuff' is the definition.
(stuff can mean to defeat or overcome)
'provided sent back before lunch?' is the wordplay.
'provided' becomes 'fed' (feeding is a kind of providing).
'sent back' shows that the letters should be reversed in order.
'before' is a charade indicator (letters next to each other).
'lunch?' becomes 'eat' (to lunch can mean to eat lunch).
'fed' in reverse letter order is 'def'.
'def'+'eat'='DEFEAT'
(Other definitions for defeat that I've seen before include "Rout" , "Beat (someone)" , "Overwhelm" , "Beat, win against" , "Win over, frustrate" .)