School cereal accompanies unknown mush (8)
I believe the answer is:
schmaltz
'mush' is the definition.
(I've seen this in another clue)
'school cereal accompanies unknown' is the wordplay.
'school' becomes 'sch'.
'cereal' becomes 'malt' (malt is a kind of cereal).
'accompanies' means one lot of letters go next to another.
'unknown' becomes 'z' (algebra).
'sch'+'malt'+'z'='SCHMALTZ'
(Other definitions for schmaltz that I've seen before include "Jewish word meaning sugary sentimentality - has just one vowel" , "Sickly sentimentality" , "Excessive sentimentality in music or film" , "mush" , "Excessive sentimentality in art or music" .)