Knockout punch no joke creates difficulty (5)
I believe the answer is:
hitch
'difficulty' is the definition.
(hitch is a kind of difficulty)
'knockout punch no joke' is the wordplay.
'knockout' becomes 'hit' (I can't explain this - if you can you should give a lot more credence to this answer).
'no' indicates named letters should be taken away.
'joke' becomes 'pun' (pun is a type of joke).
'punch' with 'pun' taken away is 'ch'.
'hit'+'ch'='HITCH'
'creates' is the link.
Can you help me to learn more?
(Other definitions for hitch that I've seen before include "Small problem" , "knot" , "Temporary problem" , "Marry" , "Temporary obstacle" .)