Piper's destination was a converted sewer (5)
I believe the answer is:
weser
'piper's destination' is the definition.
The definition suggests a singular noun which matches the answer.
'a converted sewer' is the wordplay.
'a converted' indicates anagramming the letters.
'sewer' is an anagram of 'WESER'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for weser that I've seen before include "German runner" , "Er, sew up the German river" .)