Such equipment is not unheard of (5)
I believe the answer is:
audio
'such equipment' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'not unheard of' is the wordplay.
'not unheard' becomes 'audi' (I can't justify this - if you can you should give a lot more credence to this answer).
'of' becomes 'o''.
'audi'+'o'='AUDIO'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for audio that I've seen before include "Prefix indicating hearing or sound" , "What is heard" , "Electronic sound reproduction" , "Relating to sound or hearing" , "Denoting sound" .)