A little wallaby's malnutrition becomes very bad (7)
I believe the answer is:
abysmal
'very bad' is the definition.
(I've seen this before)
'a little wallaby's malnutrition becomes' is the wordplay.
'a little' indicates a hidden word.
'ABYSMAL' is hidden in the letters of 'wallabys malnutrition becomes'.
(Other definitions for abysmal that I've seen before include "Appallingly bad" , "Very bad or bottomless" , "Pathetic" , "Extremely bad, appalling" , "Deplorable, extremely bad - say ignorance" .)