Behind on the wagon in crash (6)
I believe the answer is:
bottom
'behind' is the definition.
('bottom' can be a synonym of 'behind')
'on the wagon in crash' is the wordplay.
'on the wagon' becomes 'tt' (I've seen this in other clues).
'in' indicates putting letters inside.
'crash' becomes 'boom' (I've seen this before).
'tt' placed within 'boom' is 'BOTTOM'.
(Other definitions for bottom that I've seen before include "Least successful position" , "dry bed" , "foot" , "Lowest of the low" , "Low point" .)