Chemical container returned about finished (5)
I believe the answer is:
nitre
'chemical' is the definition.
(I know that nitre is a type of nitrate)
'container returned about finished' is the wordplay.
'container' becomes 'tin' (tin is a kind of container).
'returned' says the letters should be written backwards.
'about finished' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'tin' reversed gives 'nit'.
'nit'+'re'='NITRE'
Can you help me to learn more?
(Other definitions for nitre that I've seen before include "fertiliser" , "explosive producer?" , "constituent of gunpowder" , "Gunpowder ingredient" , "Chemical" .)