Stomach has a medium length (5)
I believe the answer is:
gamut
'length' is the definition.
The answer and definition can be both human attributes as well as being singular nouns.
Maybe there's an association between them I don't understand?
'stomach has a medium' is the wordplay.
'stomach' becomes 'gut' (I've seen this before).
'has' means one lot of letters goes inside another.
'medium' becomes 'm' (as in UK clothing sizes).
'a'+'m'='am'
'gut' going around 'am' is 'GAMUT'.
Can you help me to learn more?
(Other definitions for gamut that I've seen before include "Ambit" , "The whole range or scope of anything" , "Full (emotional) range" , "Entire range" , "Whole range (from A to Z)" .)