Fatty apes do start to ignore bananas (7)
I believe the answer is:
adipose
'fatty' is the definition.
('adipose' can be a synonym of 'fatty')
'apes do start to ignore bananas' is the wordplay.
'start to' indicates taking the first letters.
'bananas' is an anagram indicator (informal term for mad).
The first letter of 'ignore' is 'i'.
'apes'+'do'+'i'='apesdoi'
'apesdoi' is an anagram of 'ADIPOSE'.
(Other definitions for adipose that I've seen before include "Like fatty tissue" , "Relating to fat" , "I soaped up with fatty kind of tissue" , "Containing fat" , "Like a sort of fat" .)