Tea, then bed: that would have done for James (9,4)
I believe the answer is:
gunpowder plot
'that would have done for james' is the definition.
(failed attempt to assassinate James I)
'tea then bed' is the wordplay.
'tea' becomes 'gunpowder' (type of tea).
'then' means one lot of letters go next to another.
'bed' becomes 'plot' (I've seen this in other clues).
'gunpowder'+'plot'='GUNPOWDER PLOT'
(Other definitions for gunpowder plot that I've seen before include "Guy Fawkes's scheme" , "1605 assassination attempt against James I" , "Thwarted conspiracy of 1605" , "Guy Fawkes' conspiracy" , "Sixteen zero five" .)