Two ton wagon (5)
I believe the answer is:
twain
'two' is the definition.
(I've seen this before)
'ton wagon' is the wordplay.
'ton' becomes 't'.
'wagon' becomes 'wain' (I've seen this before).
't'+'wain'='TWAIN'
(Other definitions for twain that I've seen before include "Mark --, American author" , "Kipling denied ever meeting" , "Two, formerly" , "II?" , "I want (anag) - US novelist and humorist" .)