Wet earth covering animal's lair (6)
I believe the answer is:
sodden
'wet' is the definition.
(similar in meaning)
'earth covering animal's lair' is the wordplay.
'earth covering' becomes 'sod' (sod can mean a piece of earth).
'animal's lair' becomes 'den' (I've seen this before).
'sod'+'den'='SODDEN'
(Other definitions for sodden that I've seen before include "heavy with moisture" , "Soaked to the skin" , "Waterlogged" , "Completely saturated" , "Muddy" .)