Huge rodent (mouse) left one doughnut ruined (10)
I believe the answer is:
tremendous
'huge' is the definition.
(synonyms)
'rodent mouse left one doughnut ruined' is the wordplay.
'left' is a deletion indicator.
'one doughnut' becomes 'o' (letter resembling a doughnut).
'ruined' indicates an anagram.
'rodent'+'mouse'='rodentmouse'
'rodentmouse' with 'o' removed is 'rdentmouse'.
'rdentmouse' is an anagram of 'TREMENDOUS'.
(Other definitions for tremendous that I've seen before include "Fantastic" , "Excellent; very great" , "Splendid!" , "Terrific" , "Wonderful" .)