Undersea overseer? (9)
I believe the answer is:
periscope
'overseer?' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for periscope that I've seen before include "Sub's eye?" , "Optical instrument" , "Submarine viewing aid" , "Viewing instrument in a submarine" , "Submarine's viewer" .)