Two tons taken by old farm wagon (5)
I believe the answer is:
twain
'two' is the definition.
(twain is a kind of two)
'tons taken by old farm wagon' is the wordplay.
'tons' becomes 't'.
'taken by' says to put letters next to each other.
'old farm wagon' becomes 'wain' (historical term for a cart).
't'+'wain'='TWAIN'
(Other definitions for twain that I've seen before include "Mark -, pen name of S. L. Clemens" , "Two items of the same kind and well-known author" , "Two, formerly" , "the two destined never to meet?" , "that was a low number" .)