Behind tramp (3)
I believe the answer is:
bum
This clue is a double definition.
'behind' is the first definition.
('bum' can be a synonym of 'behind')
'tramp' is the second definition.
('bum' can be a synonym of 'tramp')
(Other definitions for bum that I've seen before include "Cadge" , "False" , "American tramp or English bottom" , "US tramp" , "American hobo or vulgar backside" .)