Old horse brought back by mineral plant (7)
I believe the answer is:
oregano
'mineral plant' is the definition.
'oregano' can be an answer for 'plant' (oregano is a kind of plant). I'm not sure about the 'mineral' bit.
'old horse brought back by' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'horse' becomes 'onager' (I can't justify this - if you can you should give a lot more credence to this answer).
'brought back' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'onager' reversed gives 'regano'.
'o'+'regano'='OREGANO'
Can you help me to learn more?
(Other definitions for oregano that I've seen before include "herbal preparation" , "Pizza herb" , "aromatic plant" , "Herb related to marjoram" , "Kitchen herb" .)