Substance in meat to ripen bananas (7)
I believe the answer is:
protein
'substance in meat' is the definition.
(I've seen this before)
'to ripen bananas' is the wordplay.
'bananas' indicates an anagram (informal term for mad).
'to'+'ripen'='toripen'
'toripen' with letters rearranged gives 'PROTEIN'.
(Other definitions for protein that I've seen before include "Essential compound of food" , "Important part of diet" , "Combination of amino acids in living matter" , "complex compound" , "A body chemical; pointer (anag.)" .)