Rue tedium, developing a form of hydrogen (9)
I believe the answer is:
'a form of hydrogen' is the definition.
I can't judge whether this defines the answer.
'rue tedium developing' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'ruetedium' with letters rearranged gives 'DEUTERIUM'.
Can you help me to learn more?