Mysterious tomb is extremely idiosyncratic (7)
I believe the answer is:
cryptic
'mysterious' is the definition.
(synonyms)
'tomb is extremely idiosyncratic' is the wordplay.
'tomb' becomes 'crypt' (crypt is a kind of tomb).
'is extremely' means to remove the middle letters (letters on the extremes of the word are used).
'idiosyncratic' with its middle removed is 'ic'.
'crypt'+'ic'='CRYPTIC'
(Other definitions for cryptic that I've seen before include "Obscure or puzzling (as our neighbour Crossheir)" , "Like some crosswords" , "Type of crossword clue (but not this one!)" , "this style?" , "Mysteriously obscure" .)