Postulate hydrogen content of meteor swarm (7)
I believe the answer is:
theorem
This clue could be a double definition.
'postulate hydrogen' is the first definition.
'theorem' can be an answer for 'postulate' (thesaurus). I am not sure about the 'hydrogen' bit.
'content of meteor swarm' is the second definition.
'theorem' can be an answer for 'content' (theorem is a kind of content). I'm not sure about the 'of meteor swarm' bit.
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "this has been established" , "Geometric proposition" , "Proven hypothesis" , "Mathematical formula" , "Proven proposition" .)