Mysterious tomb -- Inca, oddly (7)
I believe the answer is:
cryptic
'mysterious' is the definition.
('cryptic' can be a synonym of 'mysterious')
'tomb inca oddly' is the wordplay.
'tomb' becomes 'crypt' (crypt is a kind of tomb).
'oddly' means one should take alternating letters.
The alternating letters of 'inca' are 'ic'.
'crypt'+'ic'='CRYPTIC'
(Other definitions for cryptic that I've seen before include "Mysteriously obscure" , "Obscure or puzzling (as our page neighbour Crosaire)" , "this style?" , "Esoteric" , "Mysterious, obscure" .)