Paid off, by the end of the year, which is quick (5)
I believe the answer is:
rapid
'is quick' is the definition.
('rapid' can be a synonym of 'quick')
'paid off by the end of the year' is the wordplay.
'off' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'the end of' says to take the final letters.
The last letter of 'year' is 'r'.
'paid' is an anagram of 'apid'.
'apid' put after 'r' is 'RAPID'.
'which' acts as a link.
(Other definitions for rapid that I've seen before include "Showing pace" , "Expeditious" , "Brisk" , "Electrical connector" , "Flying" .)