Tart's mega upset by plan (9)
'plan' is the definition.
(stratagem is a kind of plan)
'tart's mega upset' is the wordplay.
'upset' is an anagram indicator.
'tarts'+'mega'='tartsmega'
'tartsmega' with letters rearranged gives 'STRATAGEM'.
'by' is the link.
(Other definitions for stratagem that I've seen before include "could be a trick" , "Clever scheme" , "Scheme to outwit an opponent" , "Device" , "A cunning plan or scheme" .)
But I've got no explanation for this one! Could you help me understand?