Rouse a knight about the start of the tourney (4)
I believe the answer is:
stir
'rouse' is the definition.
(I've seen this in another clue)
'a knight about the start of the tourney' is the wordplay.
'a knight' becomes 'sir' (a knight has the title Sir).
'about' is an insertion indicator.
'the start of' suggests taking the first letters.
The initial letter of 'tourney' is 't'.
'sir' enclosing 't' is 'STIR'.
(Other definitions for stir that I've seen before include "jail" , "Act as an agitator" , "Commotion - prison" , "Excite emotionally" , "Nick" .)