Begins with a sweet pastry on board (6)
I believe the answer is:
starts
'begins' is the definition.
('start' can be a synonym of 'begin')
'a sweet pastry on board' is the wordplay.
'a sweet pastry' becomes 'tart' (type of pastry. I am not sure about the 'sweet' bit.).
'on' is an insertion indicator.
'board' becomes 'ss' (I've seen this in other clues).
'tart' put into 'ss' is 'STARTS'.
'with' is the link.
Can you help me to learn more?
(Other definitions for starts that I've seen before include "origins?" , "jerks" , "Commences" , "Begins or causes to begin" , "Begins or flinches" .)