Key rings start to oxidise on the surface (6)
I believe the answer is:
afloat
'on the surface' is the definition.
(I've seen this before)
'key rings start to oxidise' is the wordplay.
'key' becomes 'a flat' (musical key).
'rings' is an insertion indicator (some letters are ringed or contained by others).
'start to' indicates taking the first letters.
The initial letter of 'oxidise' is 'o'.
'aflat' going around 'o' is 'AFLOAT'.
(Other definitions for afloat that I've seen before include "Resting on water" , "Still solvent" , "On the surface" , "At sea, on the water" , "On board a ship or boat" .)