Tea's first, cereal second (5)
I believe the answer is:
trice
'second' is the definition.
(both can mean a very short amount of time)
'tea's first cereal' is the wordplay.
'tea's first' becomes 't' (rst letter of 'tea').
'cereal' becomes 'rice' (rice is a kind of cereal).
't'+'rice'='TRICE'
(Other definitions for trice that I've seen before include "Very short moment" , "In a -- means very quickly" , "Old use for three times" , "Short time" , "Flash" .)