I'm going downhill, entertaining very lazy so-and-so (6)
I believe the answer is:
skiver
'lazy so-and-so' is the definition.
'i'm going downhill entertaining very' is the wordplay.
'i'm going downhill' becomes 'skier' (someone who skis).
'entertaining' is an insertion indicator.
'very' becomes 'v' (abbreviation).
'skier' enclosing 'v' is 'SKIVER'.
(Other definitions for skiver that I've seen before include "Duty dodger" , "Work avoider" , "One shirking duty" , "One who avoids work" , "Shirker" .)