Cereal stirred below temperature becoming syrup (7)
I believe the answer is:
treacle
'syrup' is the definition.
(treacle is a kind of syrup)
'cereal stirred below temperature' is the wordplay.
'stirred' is an anagram indicator.
'below' means one lot of letters go next to another.
'temperature' becomes 't' (abbreviation).
'cereal' is an anagram of 'reacle'.
'reacle' put after 't' is 'TREACLE'.
'becoming' acts as a link.
(Other definitions for treacle that I've seen before include "Sticky fluid" , "Sweet, black viscous substance" , "Syrup-like substance" , "cloying stuff" , "Thick syrup from sugar cane" .)