A tool may be observed (3)
I believe the answer is:
saw
This clue is a double definition.
'a tool' is the first definition.
(I know that saw is a type of power tool)
'be observed' is the second definition.
(seen)
'may' acts as a link.
(Other definitions for saw that I've seen before include "Was turned into a cutting tool" , "Saying; cut wood" , "Old saying - cutting kind" , "Viewed; adage" , "item in tool kit" .)