River-mouth pollution requiring switch to other side (5)
I believe the answer is:
firth
'side' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for firth that I've seen before include "Arm of the sea - actor Colin" , "Scottish river mouth" , "Scottish sea-inlet" , "Narrow sea inlet in Scotland" , "Estuary" .)