Guess correctly that you mustn’t knock off (3,2)
I believe the answer is:
hit on
'guess correctly that you mustn't' is the definition.
The definition and answer are not the same part of speech.
'knock off' is the wordplay.
'knock' becomes 'hit' (synonyms).
'off' becomes 'on' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'hit'+'on'='HIT ON'
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" .)