Treat your old well (7)
I believe the answer is:
healthy
'well' is the definition.
(associated in meaning)
'treat your old' is the wordplay.
'treat' becomes 'heal' (give medical treatment).
'your old' becomes 'thy' ('thy' historically meant 'your').
'heal'+'thy'='HEALTHY'
(Other definitions for healthy that I've seen before include "In good physical or mental shape" , "Physically sound" , "Hale, fit" , "Fit, in sound physical shape" , "Fit, undiseased" .)