Tons on wagon? A couple (5)
I believe the answer is:
twain
'a couple' is the definition.
('twain' can be a synonym of 'couple')
'tons on wagon?' is the wordplay.
'tons' becomes 't'.
'on' means one lot of letters go next to another.
'wagon?' becomes 'wain' (wain is a kind of wagon).
't'+'wain'='TWAIN'
(Other definitions for twain that I've seen before include "Two (arch.)" , "A pen-name" , "that was a low number" , "Archaic term for two" , "two in history" .)