Tethers donkey; always a drag to do this
I believe the answer is:
chain-smoke
'always a drag to do this' is the definition.
The definition suggests a singular noun but the answer is not.
'tethers donkey' is the wordplay.
'tethers' becomes 'chains' (I can't explain this - if you can you should give a lot more credence to this answer).
'donkey' becomes 'moke' (moke is a kind of donkey).
'chains'+'moke'='CHAIN-SMOKE'
Can you help me to learn more?
(Other definitions for chain-smoke that I've seen before include "Light one fag after another" , "Puff non-stop" , "Get through cigarettes one after another" , "Get through fags one after the other" , "Keep dragging" .)