Food additive spoiling muesli that is eaten by Father (10)
I believe the answer is:
emulsifier
'food additive' is the definition.
(type of food additive)
'spoiling muesli that is eaten by father' is the wordplay.
'spoiling' indicates an anagram.
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'eaten by' is an insertion indicator.
'father' becomes 'fr' (abbreviation as in a priest's title).
'ie' put into 'fr' is 'fier'.
'muesli' is an anagram of 'emulsi'.
'emulsi'+'fier'='EMULSIFIER'
(Another definition for emulsifier that I've seen is " Chemical agent".)