Put back one material on top of another (7)
I believe the answer is:
replace
'put back' is the definition.
(I know that put back can be written as replace)
'one material on top of another' is the wordplay.
'one material' becomes 'rep' (rep is a kind of material).
'on top' says to put letters next to each other (in a down clue, some letters go above others).
'of another' becomes 'lace' (I can't explain this - if you can you should give a lot more credence to this answer).
'rep'+'lace'='REPLACE'
Can you help me to learn more?
(Other definitions for replace that I've seen before include "Substitute for (someone)" , "Put back in original position" , "Substitute, supersede" , "Provide a substitute for" , "Put back where you found it" .)