Herbicide makes a chap a goner, unfortunately (5,6)
I believe the answer is:
agent orange
'herbicide' is the definition.
(I have seen 'A herbicide' mean 'agent orange' so perhaps 'herbicide' could also mean 'agent orange')
'a chap a goner unfortunately' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues).
'unfortunately' indicates anagramming the letters.
'a'+'goner'='agoner'
'agoner' with letters rearranged gives 'orange'.
'a'+'gent'+'orange'='AGENT ORANGE'
'makes' is the link.
(Other definitions for agent orange that I've seen before include "What defoliates" , "Poisonous chemical" , "Controversial weapon" , "This takes leaves off" , "Defoliant used in the Vietnam war" .)