Stomach has a medium scope (5)
I believe the answer is:
gamut
'scope' is the definition.
(gamut is a kind of scope)
'stomach has a medium' is the wordplay.
'stomach' becomes 'gut' (I've seen this before).
'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 "Complete extent or range" , "The whole range of, say emotions" , "Ambit" , "Compass, scale" , "Complete range or scale" .)