Substituted one material with another — hint of decay? (8)
I believe the answer is:
replaced
'substituted one material' is the definition.
'replaced' can be an answer for 'substituted' ('replace' can be a synonym of 'substitute'). I'm unsure of the 'one material' bit.
'with another hint of decay?' is the wordplay.
'with' says to put letters next to each other.
'another' becomes 're' (I've seen this in other clues).
'hint of decay?' becomes 'placed' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'placed' is 'REPLACED'.
Can you help me to learn more?
(Other definitions for replaced that I've seen before include "Led caper (anag.)" , "Substituted, supplanted" , "Put back in position" , "Substituted, exchanged" .)