To deprive a chap of a companion, mate, is a horrifying thing to do (5)
I believe the answer is:
appal
'a companion mate is a horrifying thing to do' is the definition.
I can't judge whether this definition defines the answer.
'to deprive a chap' is the wordplay.
'to deprive' becomes 'app' (I can't explain this - if you can you should give a lot more credence to this answer).
'a chap' becomes 'Al' (man's name).
'app'+'al'='APPAL'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for appal that I've seen before include "Put at rest (suspicion)" , "Cause to be unpleasantly surprised" , "exhibiting dismay" , "Scandalise" , "Greatly dismay" .)