Setter eats old stew alone (8)
I believe the answer is:
isolated
'alone' is the definition.
('isolated' can be similar in meaning to 'alone')
'setter eats old stew' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'stew' indicates anagramming the letters.
'eats'+'old'='eatsold'
'eatsold' with letters rearranged gives 'solated'.
'i'+'solated'='ISOLATED'
(Other definitions for isolated that I've seen before include "Placed alone" , "hard to find" , "kept apart from others" , "Remote, set apart" , "Single" .)