At Eton, I bunged up drain (3,4)
I believe the answer is:
eat into
'drain' is the definition.
(I've seen this in another clue)
'at eton i bunged up' is the wordplay.
'bunged up' indicates an anagram.
'at'+'eton'+'i'='atetoni'
'atetoni' with letters rearranged gives 'EAT INTO'.
(Other definitions for eat into that I've seen before include "Consume (resources)" , "run down" , "start wearing out" , "Etch" , "Corrode (something)" .)