Overseeing helper? (9)
I believe the answer is:
periscope
'helper?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I cannot understand how the rest of the clue works.
(Other definitions for periscope that I've seen before include "Sub's eye?" , "tube with mirrors" , "Optical instrument" , "Submarine's viewer" , "overseer" .)