Eye drop (4)
I believe the answer is:
tear
'eye drop' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tear that I've seen before include "Secretion of the lacrymal glands" , "A drop of sadness" , "A drop of optical moisture" , "Optical sign of sorrow" , "Droplet; rip" .)