As behind, knock off after finishing off last of work (7)
I believe the answer is:
arsenic
'as' is the definition.
(As is the chemical symbol for arsenic)
'behind knock off after finishing off last of work' is the wordplay.
I cannot really understand how this works, but
'behind' could be 'arse' (both can mean the buttocks) and 'arse' is found in the answer.
The remaining letters 'nic' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Brittle grey-green semi-metal, atomic number 33" , "Element with many poisonous compounds" , "Ratsbane" , "killer" , "Famous stage comedy ''. . . . . . . and Old Lace''" .)