Begins pastries after son (6)
I believe the answer is:
starts
'begins' is the definition.
('start' can be a synonym of 'begin')
'pastries after son' is the wordplay.
'pastries' becomes 'tarts' (tart is a kind of pastry).
'after' means one lot of letters go next to another.
'son' becomes 's'.
'tarts' after 's' is 'STARTS'.
(Other definitions for starts that I've seen before include "origins?" , "Reacts in shock" , "begins or recoils" , "Begins or flinches" , "Makes sudden movement from pain or surprise" .)