Forceful chimp ate bananas (8)
I believe the answer is:
emphatic
'forceful' is the definition.
(I know that forceful can be written as emphatic)
'chimp ate bananas' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'chimp'+'ate'='chimpate'
'chimpate' with letters rearranged gives 'EMPHATIC'.
(Other definitions for emphatic that I've seen before include "Forceful - pronounced" , "Impact he combined with special stress" , "Forceful and definite" , "Striking" , "Definite and clear" .)