Get greater mixture of letters in general (7)
I believe the answer is:
enlarge
'get greater' is the definition.
(to enlarge is to get or make bigger)
'mixture of letters in general' is the wordplay.
'mixture of letters in' indicates anagramming the letters.
'general' anagrammed gives 'ENLARGE'.
(Other definitions for enlarge that I've seen before include "Describe in more detail" , "Magnify in general way" , "Expatiate" , "Dilate" , "Widen" .)