Corrode a metal in periodically scratched section (3,4)
I believe the answer is:
eat into
'corrode' is the definition.
(I've seen this in another clue)
'a metal in periodically scratched section' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'in' indicates putting letters inside.
'periodically scratched' indicates alternate letters (letters scratched out periodically).
The alternating letters of 'section' are 'eto'.
'a'+'tin'='atin'
'atin' placed inside 'eto' is 'EAT INTO'.
(Other definitions for eat into that I've seen before include "start wearing out" , "run down" , "Corrode (something)" , "Consume (resources)" , "Etch" .)