Gal and mama prepared a mixture (7)
I believe the answer is:
amalgam
'a mixture' is the definition.
(I know that amalgam is a type of alloy)
'gal and mama prepared' is the wordplay.
'and' is a charade indicator (letters next to each other).
'prepared' indicates an anagram.
'gal' put after 'mama' is 'mamagal'.
'mamagal' anagrammed gives 'AMALGAM'.
(Other definitions for amalgam that I've seen before include "Blend of diverse things" , "Mixture/blend" , "Alloy of mercury and another metal" , "A soft pliable mixture" , "Mixture (for dental fillings)" .)