Large part of day filled with a ferment (9)
I believe the answer is:
leavening
'ferment' is the definition.
(thesaurus)
'large part of day filled with a' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'part of day' becomes 'evening' (latter part of the day).
'filled with' means one lot of letters goes inside another.
'evening' enclosing 'a' is 'eavening'.
'l'+'eavening'='LEAVENING'
(Other definitions for leavening that I've seen before include "Making to rise" , "Influential agent" , "eg Yeast" , "instigator of rising" , "Riser" .)