Fours swap places in tremendous location on the Thames (8)
I believe the answer is:
kingston
'fours swap' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'places in tremendous location on the thames' is the wordplay.
I cannot really understand how this works, but
'in' is found in the answer.
'on' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for kingston that I've seen before include "Royal borough on river Thames" , "Capital of Jamaica" , "island capital" , "Jamaican capital" .)