Guess correctly that you mustn't knock off (3,2)
I believe the answer is:
hit on
'guess' is the definition.
Both the answer and definition are verbs in their base form.
Maybe they are linked in a way I don't understand?
'correctly that you mustn't knock off' is the wordplay.
I cannot quite see 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 see.
This explanation may well be incorrect...
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" .)