Mineral water: tiny upturned bottles (5)
I believe the answer is:
nitre
'mineral' is the definition.
(term for potassium nitrate)
'water tiny upturned bottles' is the wordplay.
'bottles' indicates the answer is hidden within the clue (bottling is a kind of containing).
'upturned' says the letters should be written backwards.
'ertin' can be found hidden inside 'water tiny'.
'ertin' back-to-front is 'NITRE'.
(Other definitions for nitre that I've seen before include "Gunpowder ingredient" , "constituent of gunpowder" , "Mineral used in gunpowder" , "Otherwise saltpetre" , "Inert kind of saltpetre" .)