Old couple back for treatment, on the wagon (5)
I believe the answer is:
twain
'old couple' is the definition.
(I've seen this before)
'on the wagon' is the wordplay.
'on' is a charade indicator (letters next to each other).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'wagon' becomes 'wain' (wain is a kind of wagon).
't' put next to 'wain' is 'TWAIN'.
'back for treatment' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for twain that I've seen before include "Archaic word for two" , "Mark ..... created Tom Sawyer" , "Mark --, American author" , "Pen name of Samuel Langhorne Clemens, d. 1910)" , "Archaic term for two" .)