Swampy planet has some hydrogen (6)
I believe the answer is:
marshy
'swampy' is the definition.
(I know that swampy can be written as marshy)
'planet has some hydrogen' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'has' says to put letters next to each other.
'some hydrogen' becomes 'hy' (I can't explain this - if you can you should give a lot more credence to this answer).
'mars'+'hy'='MARSHY'
Can you help me to learn more?
(Other definitions for marshy that I've seen before include "(Of land) waterlogged" , "like Slough" , "Damp and low-lying" , "Wet and low-lying" , "Boggy; waterlogged" .)