Starts to tune horn up - dreary dull sound (4)
I believe the answer is:
thud
'dull sound' is the definition.
(I've seen this before)
'starts to tune horn up dreary' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'tune horn up dreary' is 'THUD'.
(Other definitions for thud that I've seen before include "Dull heavy impact" , "Dull sound of impact" , "Sound made by impact of heavy objects" , "Sound of a heavy fall" , "Clunk" .)