Maiden upset with gut size (9)
I believe the answer is:
magnitude
'size' is the definition.
(I know that size can be written as Magnitude)
'maiden upset with gut' is the wordplay.
'upset with' is an anagram indicator.
'maiden' with its letters mixed up with 'gut' makes 'MAGNITUDE'.
(Other definitions for magnitude that I've seen before include "Dimensions" , "(A star's) brightness" , "Eating mud (anag.)" , "Relative importance" , "Largeness or size" .)