Instrument for drawing right angles (1-6)
I believe the answer is:
t-square
'instrument for drawing right angles' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for t-square that I've seen before include "Draughtsman's tool" , "Instrument for drawing right angles" , "Draftsman's instrument" , "Without denizens" , "Instrument used by draftsmen to draw parallel lines" .)