Possible Clues:

• Ape powerless to trap man, one such becoming killer in Vietnam
• This takes leaves off different varieties of gean on either side of hill
• Powerful herbicide with a colorful name
• Toxic defoliant used in Vietnam
• One needs knowledge to classify herbicide
• ... following a man round to get defoliant
• Nerve gas makes a chap a goner, unfortunately
• Toxic herbicide
• A fellow gets round stretch of open country offering reason for lack of greenery?
• Highly poisonous herbicide used by US forces during the Vietnam War