Begin small pastry dish (5)
I believe the answer is:
'begin' is the definition.
(I know that begin can be written as start)
'small pastry dish' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pastry dish' becomes 'tart' (type of pastry product).
(Other definitions for start that I've seen before include "Begin with sudden movement of surprise" , "Begin; flinch" , "Begin to recoil?" , "lead" , "Beginning" .)