Mend your old-fashioned well (7)
I believe the answer is:
healthy
'well' is the definition.
(similar in meaning)
'mend your old-fashioned' is the wordplay.
'mend' becomes 'heal' (synonyms).
'your old-fashioned' becomes 'thy' (archaic word for 'your').
'heal'+'thy'='HEALTHY'
(Other definitions for healthy that I've seen before include "In good physical or mental shape" , "Fit, in sound physical shape" , "Physically sound" , "Physically well" , "In good shape, not ill" .)