Yellow mineral companion found inside (5)
I believe the answer is:
ochre
'yellow' is the definition.
(ochre is a kind of yellow)
'mineral companion found inside' is the wordplay.
'mineral' becomes 'ore' (ore is a kind of mineral).
'companion' becomes 'ch' (Companion of Honour).
'found inside' indicates putting letters inside.
'ore' placed around 'ch' is 'OCHRE'.
(Other definitions for ochre that I've seen before include "Orange-yellow colour" , "Light yellow-brown colour" , "yellowy-orange" , "Earth coloured pigment" , "Chore (anag) - pale brownish-yellow" .)