Excess muscle trimmed (4)
I believe the answer is:
glut
'excess' is the definition.
(glut can mean an excess of something)
'muscle trimmed' is the wordplay.
'muscle' becomes 'glute' (short for gluteus).
'trimmed' means to remove the last letter (the last letter is trimmed off).
'glute' with its final letter taken off is 'GLUT'.
(Other definitions for glut that I've seen before include "stuff" , "A surfeit as of crops" , "Excess, surfeit" , "Surfeit or over-supply" , "Superfluity" .)