Set up trap in matching puzzles (7)
I believe the answer is:
enigmas
'puzzles' is the definition.
(an enigma is a puzzle or mystery)
'set up trap in matching' is the wordplay.
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'trap' becomes 'gin' (gin trap is a kind of animal trap).
'in' means one lot of letters goes inside another.
'matching' becomes 'same' (I've seen this in another clue).
'gin' inserted inside 'same' is 'samgine'.
'samgine' in reverse letter order is 'ENIGMAS'.
(Other definitions for enigmas that I've seen before include "Puzzles - mysteries" , "'Mysteries, riddles (7)'" , "Problems" , "'Puzzles, riddles (7)'" , "They're mysterious or difficult to understand" .)