Look, yours truly accepts single's solitary (8)
I believe the answer is:
lonesome
'solitary' is the definition.
(I know that solitary can be written as lonesome)
'look yours truly accepts single's' is the wordplay.
'look' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'yours truly' becomes 'me' (the person that set this crossword).
'accepts' means one lot of letters goes inside another.
'single's' becomes 'one's' (a single thing - only one).
'lo'+'me'='lome'
'lome' going around 'ones' is 'LONESOME'.
(Other definitions for lonesome that I've seen before include "Sad and lonely" , "On one's own" , "Solitary (chiefly US)" , "Solitary (mainly US)" , "Are you -- tonight (Elvis)" .)