Boggy, extremely heavy on planet (6)
I believe the answer is:
marshy
'boggy' is the definition.
('marshy' can be a synonym of 'boggy')
'extremely heavy on planet' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'on' means one lot of letters go next to another.
'planet' becomes 'mars' (Mars is an example).
'heavy' with its centre removed is 'hy'.
'hy' put after 'mars' is 'MARSHY'.
(Other definitions for marshy that I've seen before include "Wet and low-lying" , "Damp and low-lying" , "(Of ground) wet" , "Waterlogged ground" , "Soggy" .)