Tripe on toast is source of this (7)
I believe the answer is:
protein
'source of this' is the definition.
The definition suggests a singular noun which matches the answer.
'tripe on toast' is the wordplay.
'toast' is an anagram indicator (toast can mean damaged or broken).
'tripe'+'on'='tripeon'
'tripeon' is an anagram of 'PROTEIN'.
'is' acts as a link.
(Other definitions for protein that I've seen before include "A body chemical; pointer (anag.)" , "Combination of amino acids in living matter" , "Not ripe (anag.)" , "Body-builder" , "Dietary requirement" .)