Finish breakfast with the cereal of the moment! (5)
I believe the answer is:
trice
'the moment' is the definition.
(thesaurus)
'finish breakfast with the cereal' is the wordplay.
'finish' says to take the final letters.
'with' means one lot of letters go next to another.
'the cereal' becomes 'rice' (rice is a kind of cereal**).
The last letter of 'breakfast' is 't'.
't'+'rice'='TRICE'
'of' acts as a link.
(Other definitions for trice that I've seen before include "Very brief moment" , "Old use for three times" , "Next to no time" , "Flash" , "Very short moment" .)