Begin small flan (5)
I believe the answer is:
start
'begin' is the definition.
(I know that begin can be written as start)
'small flan' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'flan' becomes 'tart' (I've seen this before).
's'+'tart'='START'
(Other definitions for start that I've seen before include "Begin with sudden movement of surprise" , "Set in motion" , "open" , "Origin" , "Commence" .)