Guess correctly that you mustn't knock off (3,2)
I believe the answer is:
hit on
'guess' is the definition.
Both the definition and answer are verbs in their base form.
Perhaps you can see an association between them that I can't see?
'correctly that you mustn't knock off' is the wordplay.
I cannot really understand how this works, but
'knock' could be 'hit' (synonyms) and 'hit' is located in the answer.
The remaining letters 'on' 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 hit on that I've seen before include "Make advances to (US)" , "Discover, think of" , "try a pass?" , "Serendipitously find" , "Discovered suddenly or, in modern slang, made advances towards" .)