Quietly direct what courtroom defendant must do (5)
I believe the answer is:
plead
'what courtroom defendant must do' is the definition.
(how do you plead)
'quietly direct' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'direct' becomes 'lead' (direct can mean to lead someone somewhere).
'p'+'lead'='PLEAD'
(Other definitions for plead that I've seen before include "Say one is guilty or not guilty" , "Beg, implore" , "Claim; entreat" , "Argue (a position in court)" , "advocate in court" .)