A long walk with a hot-headed fellow (4)
I believe the answer is:
hike
'a long walk' is the definition.
(a hike is a long walk)
'with a hot-headed fellow' is the wordplay.
'with' says to put letters next to each other.
'a hot' becomes 'h' (abbreviation eg on taps).
'headed fellow' becomes 'ike' (I can't justify this - if you can you should believe this answer much more).
'h' put next to 'ike' is 'HIKE'.
Can you help me to learn more?
(Other definitions for hike that I've seen before include "Travel by Shank's Mare" , "physical exercise" , "Long ramble" , "Long march; increase" , "(Price) increase; walk" .)