Origin of bunk discussed (5)
I believe the answer is:
birth
'origin' is the definition.
(I've seen this in another clue)
'bunk discussed' is the wordplay.
'bunk' becomes 'berth' (synonyms).
'discussed' indicates a 'sounds like' (homophone) clue.
'berth' sounds like 'BIRTH'.
'of' is the link.
(Other definitions for birth that I've seen before include "Parentage" , "origin" , "Nativity" , "Delivery (of child)" , "Beginning; ancestry" .)