Take an oath in sideshow's first and final hearing (5)
I believe the answer is:
swear
'take an oath' is the definition.
(I've seen this in another clue)
'sideshow's first and final hearing' is the wordplay.
'sideshow's first' becomes 's' (wst letter of 'sideshow').
'and' says to put letters next to each other.
'final' becomes 'w' (I can't explain this - if you can you should give a lot more credence to this answer).
'hearing' becomes 'ear' (ear is a kind of hearing).
's'+'w'+'ear'='SWEAR'
'in' is the link.
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Eff and blind" , "Solemnly promise" , "Avow" , "Curse" , "Vow; blaspheme" .)