Trace of sewage found in abandoned septic tank (7)
I believe the answer is:
cesspit
'tank' is the definition.
(I know that Cesspit is a type of cistern)
'trace of sewage found in abandoned septic' is the wordplay.
'trace of' says to take the initial letters.
'found in' indicates putting letters inside.
'abandoned' is an anagram indicator (the original order is abandoned).
The initial letter of 'sewage' is 's'.
'septic' anagrammed gives 'cespit'.
's' inserted inside 'cespit' is 'CESSPIT'.
(Other definitions for cesspit that I've seen before include "Sewage storer" , "Place for filthy water" , "means of waste disposal" , "Foul and squalid place" , "Sewage receptacle" .)