River quietens with injection of hydrogen (6)
I believe the answer is:
thames
'river' is the definition.
(English river)
'quietens with injection of hydrogen' is the wordplay.
'quietens' becomes 'tames' (I can't justify this - if you can you should give a lot more credence to this answer).
'with injection of' is an insertion indicator (some letters injected in).
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'tames' placed around 'h' is 'THAMES'.
Can you help me to learn more?
(Other definitions for thames that I've seen before include "London river" , "capital banker" , "Shipping forecast area" , "Windsor's river" , "One going to live abroad" .)