Rent finally collected by each foreign letter (3)
I believe the answer is:
eta
'foreign letter' is the definition.
(Greek letter)
'rent finally collected by each' is the wordplay.
'finally' suggests the final letters.
'collected by' indicates putting letters inside.
'each' becomes 'ea' (abbreviation).
The last letter of 'rent' is 't'.
't' placed inside 'ea' is 'ETA'.
(Other definitions for eta that I've seen before include "Militant Basque organisation - Greek letter" , "indicating when flight is due" , "Basque separatist movement" , "When one is due" , "Letter from abroad" .)