Time cereal was instant! (5)
I believe the answer is:
trice
'instant' is the definition.
(both can mean a short amount of time)
'time cereal' is the wordplay.
'time' becomes 't' (abbreviation).
'cereal' becomes 'rice' (rice is a kind of cereal).
't'+'rice'='TRICE'
'was' is the link.
(Other definitions for trice that I've seen before include "Very brief moment" , "Instant of time" , "Second" , "Very short moment" , "Flash" .)