Mineral gunners kept in container (5)
I believe the answer is:
borax
'mineral' is the definition.
(I've seen this before)
'gunners kept in container' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'kept in' means one lot of letters goes inside another.
'container' becomes 'box' (box is a kind of container**).
'ra' going inside 'box' is 'BORAX'.
(Other definitions for borax that I've seen before include "White salt" , "Hydrated mineral" .)