Excess is left in the alimentary canal (4)
I believe the answer is:
glut
'excess' is the definition.
(glut can mean an excess of something)
'left in the alimentary canal' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in the' indicates putting letters inside.
'alimentary canal' becomes 'gut' (I've seen this before).
'l' placed within 'gut' is 'GLUT'.
'is' is the link.
(Other definitions for glut that I've seen before include "Excessively abundant supply" , "Overabundance" , "Superfluity" , "Excessive abundance" , "Flood" .)