Mysterious nun crazy about king now (7)
I believe the answer is:
unknown
'mysterious' is the definition.
(I've seen this before)
'nun crazy about king now' is the wordplay.
'crazy' indicates anagramming the letters.
'about' is an insertion indicator.
'king' becomes 'K' (abbreviation for king in chess).
'nun' anagrammed gives 'unn'.
'k'+'now'='know'
'unn' enclosing 'know' is 'UNKNOWN'.
(Other definitions for unknown that I've seen before include "A number to be found" , "Not recognized" , "Opposite of 1 Down" , "Unidentified, strange" , "Not recognised by anyone" .)