Astronomer's instrument (9)
I believe the answer is:
telescope
'astronomer's instrument' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for telescope that I've seen before include "It magnifies the image of distant objects" , "contract" , "Magnifier of distant images" , "Instrument to magnify the image of distant objects" , "Magnifier of distant objects" .)