He knocks you out with an extension to showroom walls (7)
I believe the answer is:
sandman
'he knocks you out with an extension to showroom walls' is the definition.
The definition suggests a singular noun which matches the answer.
'he knocks you out with' is the wordplay.
I cannot really see how this works, but
'he' could be 'man' (I've seen this in another clue) and 'man' is present in the answer.
'with' could be 'and' and 'and' is located in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sandman that I've seen before include "sleepy character?" , "Imaginary person who causes children to sleep" , "Fairy who puts children to sleep" , "He comes to send children off" , "Children's bedtime fairy" .)