Monocle, basically
I believe the answer is:
lens
'monocle basically' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lens that I've seen before include "City of northern France: periscope part" , "Device to converge or diverge light" , "town in France" , "Transparent optical device" , "French town" .)