Mineral Renoir mixed with ochre initially (4,3)
I believe the answer is:
iron ore
'mineral' is the definition.
(iron ore is a kind of mineral)
'renoir mixed with ochre initially' is the wordplay.
'mixed with' indicates an anagram.
'initially' says to take the initial letters.
The first letter of 'ochre' is 'o'.
'renoir' mixed up with 'o' can make 'IRON-ORE'.
(Other definitions for iron ore that I've seen before include "source of heavy metal" , "Ferriferous mineral" , "Ferrous mineral" , "Metal-yielding mineral" , "Rock from which a metal can be extracted" .)