Can you knock it on? (3)
I believe the answer is:
tap
'can you' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'knock it on?' is the wordplay.
'knock' becomes 'ap' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'on?' says to put letters next to each other.
'ap' after 't' is 'TAP'.
Can you help me to learn more?
(Other definitions for tap that I've seen before include "indicate availability" , "A form of dance" , "One can be hot or cold?" , "Get money from, strike" , "Make use of; water from it" .)