A drop in rent (4)
I believe the answer is:
tear
I believe this clue is a double definition.
'a drop' is the first definition.
(thesaurus)
'rent' is the second definition.
('tear' can be a synonym of 'rent')
'in' is the link.
(Other definitions for tear that I've seen before include "A drop of sadness" , "A drop of optical moisture" , "Rip, pull apart" , "Lachrymal drop - rip" , "Rip - eye moisture" .)