Updated drainage system, reversing initial direction (5)
I believe the answer is:
newer
'updated drainage system' is the definition.
'newer' can be an answer for 'updated' (more up to date or new). I'm unsure of the 'drainage system' bit.
'reversing initial direction' is the wordplay.
'reversing' says the letters should be written in reverse.
'initial' becomes 'ewen' (I can't justify this - if you can you should give a lot more credence to this answer).
'direction' becomes 'r' (r is abbreviation for right).
'ewen' written backwards gives 'newe'.
'newe'+'r'='NEWER'
(Other definitions for newer that I've seen before include "Fresher, not as old" , "More modern" , "More pristine or recently invented" , "More recent" , "Fresher or more novel" .)