Stomach has a medium range (5)
I believe the answer is:
gamut
'range' is the definition.
(gamut can mean a range or variety)
'stomach has a medium' is the wordplay.
'stomach' becomes 'gut' (I've seen this in other clues).
'has' is an insertion indicator.
'medium' becomes 'm' (as in UK clothing sizes).
'a'+'m'='am'
'gut' placed around 'am' is 'GAMUT'.
(Other definitions for gamut that I've seen before include "Full range, compass" , "Ambit" , "The whole range or scope of anything" , "Entire scale" , "Entire range" .)