Blow up and down (4)
I believe the answer is:
toot
This clue is a double definition.
'blow' is the first definition.
(I've seen this before)
'up and down' is the second definition.
(Other definitions for toot that I've seen before include "Sound one's horn" , "Brief blast on a horn" , "Parp" , "Horn's sound (T, not H)" , "Noise made by car horn" .)