Reportedly why a hooter makes us cry mournfully (4)
I believe the answer is:
yowl
'cry mournfully' is the definition.
(yowl can mean to cry or wail)
'reportedly why a hooter' is the wordplay.
'reportedly' shows a homophone (sound like).
'a hooter' becomes 'owl' (owls hoot).
'why' sounds like 'y'.
'y'+'owl'='YOWL'
'makes us' acts as a link.
(Other definitions for yowl that I've seen before include "Wail" , "Wailing cry (not howl)" .)