Begin a second flan (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'a second flan' is the wordplay.
'a second' becomes 's' (common abbreviation).
'flan' becomes 'tart' (I've seen this before).
's'+'tart'='START'
(Other definitions for start that I've seen before include "Begin to recoil?" , "Commencement" , "Set in motion" , "Begin; flinch" , "Onset" .)