Take down or plug up (4)
I believe the answer is:
gulp
'take down' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'plug up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'plug' back-to-front is 'GULP'.
'or' is the link.
Can you help me to learn more?
(Other definitions for gulp that I've seen before include "Swallow in large mouthfuls" , "Audible swallow" , "Noise made when swallowing too quickly" , "Swallow nervously" , "Swallow audibly" .)