Old woman fuses hydrogen and silver (3)
I believe the answer is:
hag
'old woman' is the definition.
(I know that hag is a type of old woman)
'fuses hydrogen and silver' is the wordplay.
I cannot really understand how this works, but
'hydrogen' could be 'h' (H is the chemical symbol for hydrogen) and 'h' is located in the answer.
'silver' could be 'ag' (Ag is the chemical symbol for silver) and 'ag' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hag that I've seen before include "Witch or ugly old woman" , "Old witch-like woman" , "Crone" .)