Mineral mixture in ewer falling short (11)
I believe the answer is:
pitchblende
'mineral' is the definition.
(pitchblende is a kind of mineral)
'mixture in ewer falling short' is the wordplay.
'mixture' becomes 'blend' (blend is a kind of mixture).
'in' means one lot of letters goes inside another.
'ewer' becomes 'pitcher' (synonyms).
'falling short' means to remove the last letter.
'pitcher' with its final letter removed is 'pitche'.
'blend' placed into 'pitche' is 'PITCHBLENDE'.
(Other definitions for pitchblende that I've seen before include "Brown or black mineral containing radium" , "source of uranium" , "Uranium ore" .)