Mend your old well (7)
I believe the answer is:
healthy
'well' is the definition.
(similar in meaning)
'mend your old' is the wordplay.
'mend' becomes 'heal' (synonyms).
'your old' becomes 'thy' ('thy' historically meant 'your').
'heal'+'thy'='HEALTHY'
(Other definitions for healthy that I've seen before include "Wholesome" , "Free from infirmity or disease" , "Fit, undiseased" , "In good shape, not ill" , "Hale, fit" .)