Organ necessary in rehearsal (3)
I believe the answer is:
ear
'organ' is the definition.
(ear is a kind of organ)
'in rehearsal' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EAR' is hidden in the letters of 'rehearsal'.
'necessary' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ear that I've seen before include "one hears" , "Musical appreciation" , "Sensory organ" , "Shell-like; corn" , "Auditory organ" .)