Put one in a solution of meths, and you've got water! (6)
I believe the answer is:
thames
'put one' is the definition.
The definition suggests a singular noun which matches the answer.
'a solution of meths and you've got water' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
an anagram of 'meths' is 'thmes' which is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for thames that I've seen before include "Windsor's river" , "London flood control" , "London's river (6)" , "River at Goring" , "The Isis at Oxford" .)