Submit, clutching back of ankle joint (6)
I believe the answer is:
reefer
'joint' is the definition.
(informal term for a marijuana joint)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for reefer that I've seen before include "In slang, a marijuana cigarette" , "pot holder?" , "Nautical jacket" , "cold vehicle" , "close-fitting garment" .)