Drains made up as an afterthought (4)
I believe the answer is:
saps
'drains' is the definition.
('sap' can be a synonym of 'drain')
'up as an afterthought' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'an afterthought' becomes 'ps' (ps stands for postscript).
'as' backwards is 'sa'.
'sa'+'ps'='SAPS'
'made' acts as a link.
(Other definitions for saps that I've seen before include "More than one fool" , "Drains energy from" , "Vital juices" , "trenches" , "Coshes" .)