Cinnamon ultimately goes in deep-fried bananas, as already explained! (10)

'already explained' is the definition.
(defined or explained already)

'cinnamon ultimately goes in deep-fried bananas' is the wordplay.
'ultimately' indicates one should take the final letters.
'goes in' indicates putting letters inside.
'bananas' is an anagram indicator (bananas can informally mean crazy).
The last letter of 'cinnamon' is 'n'.
'deep'+'fried'='deepfried'
'deepfried' anagrammed gives 'predefied'.
'n' put inside 'predefied' is 'PREDEFINED'.

'as' acts as a link.