Couple, the first on the wagon (5)
I believe the answer is:
twain
'couple' is the definition.
('twain' can be a synonym of 'couple')
'the first on the wagon' is the wordplay.
'first' indicates taking the first letters.
'on' says to put letters next to each other.
'the wagon' becomes 'wain' (wain is a kind of wagon).
The first letter of 'the' is 't'.
't'+'wain'='TWAIN'
(Other definitions for twain that I've seen before include "Tom Sawyer author, d. 1910" , "Mark ___ , American writer" , "Two items of the same kind and well-known author" , "two in history" , "Mark ..... created Huckleberry Finn" .)