Make mistake in The Star Chamber finally (3)
I believe the answer is:
err
'make mistake' is the definition.
(to err is to make an error)
'the star chamber finally' is the wordplay.
'finally' indicates one should take the final letters.
The last letters of 'the star chamber' are 'ERR'.
'in' acts as a link.
(Other definitions for err that I've seen before include "Miscalculate" , "Stray from the straight and narrow" , "Make a wrong move" , "Make mistakes" , "Blunder" .)