Use horn, coming and going (4)
I believe the answer is:
toot
'use horn' is the definition.
(I've seen this before)
'and going' is the wordplay.
'and' becomes 'too'.
'going' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'too'+'t'='TOOT'
'coming' acts as a link.
Can you help me to learn more?
(Other definitions for toot that I've seen before include "Sound of a car horn" , "Parp" , "honk horn" , "Blow a horn" , "Horn's sound (T, not H)" .)