Take an oath, but show it's void before hearing (5)
I believe the answer is:
swear
'take an oath' is the definition.
(I've seen this before)
'show it's void before hearing' is the wordplay.
'it's void' suggests removing the centre (the word is empty or void of its inside letters).
'before' says to put letters next to each other.
'hearing' becomes 'ear' (ear is a kind of hearing).
'show' with its middle taken out is 'sw'.
'sw'+'ear'='SWEAR'
'but' acts as a link.
(Other definitions for swear that I've seen before include "Use indecent or blasphemous language" , "Utter obscenities or take an oath" , "not to mince one's words" , "Asseverate" , "Avow" .)