Returning American politician finds a cesspool (4)
I believe the answer is:
sump
'a cesspool' is the definition.
(I know that cesspool can be written as sump)
'returning american politician' is the wordplay.
'returning' is a reversal indicator.
'american' becomes 'US' (United States).
'politician' becomes 'MP' (Member of Parliament).
'us' reversed gives 'su'.
'su'+'mp'='SUMP'
'finds' is the link.
(Other definitions for sump that I've seen before include "oil well?" , "Cesspool" , "Pit receiving drainage" , "lowest point of mine" , "Pit collecting water/oil" .)