Extent of horrible mud-eating (9)
I believe the answer is:
magnitude
'extent' is the definition.
(I've seen this in another clue)
'horrible mud-eating' is the wordplay.
'horrible' indicates an anagram.
'mud'+'eating'='mudeating'
'mudeating' with letters rearranged gives 'MAGNITUDE'.
'of' acts as a link.
(Other definitions for magnitude that I've seen before include "Relative importance" , "Dimensions" , "(A star's) brightness" , "Largeness or size" , "Eating mud (anag.)" .)