Begin second baked dish (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'second baked dish' is the wordplay.
'second' becomes 's' (common abbreviation).
'baked dish' becomes 'tart' (tart is a type of baked item).
's'+'tart'='START'
(Other definitions for start that I've seen before include "begin an argument?" , "launch" , "Initiate" , "Begin to recoil?" , "Set in motion" .)