Passing mention had effect on nothing (4)
I believe the answer is:
obit
'passing mention' is the definition.
(I've seen this before)
'effect on nothing' is the wordplay.
'effect' becomes 'bit' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'nothing' becomes 'o' (looks like zero - 0).
'bit' put after 'o' is 'OBIT'.
'had' acts as a link.
Can you help me to learn more?
(Other definitions for obit that I've seen before include "Biography after death, in short" , "Death notice (colloq.)" , "The last word" , "Short biography of someone who has died (abbr)" , "Death notice abbreviated" .)