A setter includes trap, going over puzzle (6)
I believe the answer is:
enigma
'puzzle' is the definition.
(I've seen this before)
'a setter includes trap going over' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'includes' indicates putting letters inside.
'trap' becomes 'gin' (gin trap is a kind of animal trap).
'going over' shows that the letters should be reversed in order.
'a'+'me'='ame'
'ame' placed around 'gin' is 'amgine'.
'amgine' back-to-front is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysterious or puzzling thing" , "closed book" , "Title of Elgar's set of variations" , "cryptic puzzle!" , "Mysteriously puzzling person or thing" .)