African leaves End of Watch by King old volunteers taken aback (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

End letter of [watc] H after K (abbreviation for king in chess or playing cards), then TA (abbreviation for Territorial Army = old name for what is now the Army Reserve = military volunteers) reversed (taken aback). Leaves chewed as a psychoactive stimulant, popular in eastern Africa.