Waterlogged lair beneath turf (6)
I believe the answer is:
sodden
'waterlogged' is the definition.
(I've seen this before)
'lair beneath turf' is the wordplay.
'lair' becomes 'den' (synonyms).
'beneath' means one lot of letters go next to another (in a down clue).
'turf' becomes 'sod' (synonyms).
'den' put after 'sod' is 'SODDEN'.
(Other definitions for sodden that I've seen before include "'Soaked, sopping (6)'" , "Soaking wet" , "Thoroughly wet, soaked" , "heavy with moisture" , "Drenched, really soaked" .)