Horse in box comes back fine (7)
I believe the answer is:
elegant
'fine' is the definition.
(similar in meaning)
'horse in box comes back' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'in' is an insertion indicator.
'box' becomes 'tele' (I've seen this in other clues).
'comes back' shows that the letters should be reversed in order.
'nag' going into 'tele' is 'tnagele'.
'tnagele' backwards is 'ELEGANT'.
(Other definitions for elegant that I've seen before include "Dignified and graceful" , "Debonair, stylishly dressed" , "Refined gracefulness" , "Tasteful and refined in appearance" , "Polished" .)