Puncture bed laid on by upset journalist (6)
I believe the answer is:
debunk
'puncture' is the definition.
(debunk can mean to puncture or undermine)
'bed laid on by upset journalist' is the wordplay.
'bed' becomes 'bunk' (bunk is a kind of bed).
'laid on by' means one lot of letters go next to another (in a down clue, some letters lay on top of others).
'upset' says the letters should be written backwards.
'journalist' becomes 'ed' (short for editor).
'ed' backwards is 'de'.
'bunk' after 'de' is 'DEBUNK'.
(Other definitions for debunk that I've seen before include "Explode pretensions, myth, of" , "Reveal as false, show up" , "Disprove" , "Expose false claims" , "Expose (pretentious claims?)" .)