We should punch, knocking out fellow second (4)
I believe the answer is:
lets
'we should' is the definition.
I can't tell whether this definition defines the answer.
'punch knocking out fellow second' is the wordplay.
'punch' becomes 'stl' (I can't explain this - if you can you should give a lot more credence to this answer).
'knocking' shows that the letters should be reversed in order.
'out' indicates putting letters inside.
'fellow second' becomes 'e' (tnd letter of 'fellow').
'stl' in reverse letter order is 'lts'.
'lts' placed around 'e' is 'LETS'.
Can you help me to learn more?
(Other definitions for lets that I've seen before include "Allows or causes to" , "Yes" , "Permits" , "Rents out or allows" , "Leases" .)