Lough of questionable status, receiving last of sludge from river (6)
I believe the answer is:
liffey
'river' is the definition.
(Irish river)
'lough of questionable status receiving last of sludge' is the wordplay.
'lough' becomes 'l'.
'of questionable status' becomes 'iffy' (iffy can mean dubious or uncertain).
'receiving' indicates putting letters inside.
'last of' suggests the final letters.
The final letter of 'sludge' is 'e'.
'iffy' going around 'e' is 'iffey'.
'l'+'iffey'='LIFFEY'
'from' acts as a link.
(Other definitions for liffey that I've seen before include "Dublin river" , "Dublin's river" .)