Engineers may use it cleverly in a radio link (3)
I believe the answer is:
oil
'radio link' 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 can't see?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for oil that I've seen before include "what can be seen on iceberg" , "Grease" , "Lubricating liquid" , "painting" , "Slippery liquid" .)