Very bad bananas deter apes (9)
I believe the answer is:
desperate
'very bad' is the definition.
(desperate means very bad or urgent)
'bananas deter apes' is the wordplay.
'bananas' indicates an anagram (bananas can informally mean crazy).
'deter'+'apes'='deterapes'
'deterapes' anagrammed gives 'DESPERATE'.
(Other definitions for desperate that I've seen before include "Extremely dangerous (situation)" , "critical" , "Adept seer (anag)" , "Nearly hopeless" , "Having lost all hope" .)