Spooner's dog unwell, sloping off (8)
I believe the answer is:
downhill
'off' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'spooner's dog unwell sloping' is the wordplay.
I cannot really understand how this works, but
'unwell' could be 'ill' (similar in meaning) and 'ill' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for downhill that I've seen before include "coast here" , "With the slope" , "race" , "How one skis" , "getting worse?" .)