Planet hot and ultimately very wet underfoot (6)
I believe the answer is:
marshy
'underfoot' is the definition.
(I have seen 'treacherous underfoot' mean 'marshy' so perhaps 'underfoot' could also mean 'marshy')
'planet hot and ultimately very' is the wordplay.
'planet' becomes 'mars' (Mars is a planet).
'hot' becomes 'h' (abbreviation eg on taps).
'and' says to put letters next to each other.
'ultimately' suggests the final letters.
The final letter of 'very' is 'y'.
'mars'+'h'+'y'='MARSHY'
'wet' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for marshy that I've seen before include "Boggy, swampy" , "Long narrow mark" , "(Of ground) wet" , "treacherous underfoot" , "like Slough" .)