Ornament to corrode (4)
I believe the answer is:
fret
This clue is a double definition.
'ornament' is the first definition.
(fretting is a kind of ornamenting)
'corrode' is the second definition.
(fretting is a kind of corroding)
'to' acts as a link.
(Other definitions for fret that I've seen before include "Worry - ridge on guitar's fingerboard" , "Worry unnecessarily" , "coastal mist" , "Split up" , "Worry; or the marking on a guitar" .)