A drop in the rent (4)
I believe the answer is:
tear
I believe this clue is a double definition.
'a drop' is the first definition.
(teardrop)
'the rent' is the second definition.
('tear' can be a synonym of 'rent')
'in' acts as a link.
(Other definitions for tear that I've seen before include "Rip, sever roughly" , "Secretion of the lachrymal glands" , "A rip or cut" , "Rip the eye moistener?" , "Rip, rend" .)