One sees what's up, in the main (9)
I believe the answer is:
periscope
'main' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for periscope that I've seen before include "Submarine's viewer" , "Optical instrument" , "Viewing instrument in a submarine" , "Submarine viewing aid" , "Sub's eye?" .)