Twin full of spirit rejected such mysteries (7)
I believe the answer is:
enigmas
'mysteries' is the definition.
('enigma' can be a synonym of 'mystery')
'twin full of spirit rejected' is the wordplay.
'twin' becomes 'same'.
'full of' indicates putting letters inside.
'spirit' becomes 'gin' (I've seen this before).
'rejected' is a reversal indicator.
'same' enclosing 'gin' is 'samgine'.
'samgine' written backwards gives 'ENIGMAS'.
'such' is the link.
(Other definitions for enigmas that I've seen before include "'Puzzles, riddles (7)'" , "Puzzles - mysteries" , "'Mysteries, riddles (7)'" , "Problems" , "They're mysterious or difficult to understand" .)