Criminal going around an unknown gorge (6)
I believe the answer is:
canyon
'gorge' is the definition.
(I know that gorge can be written as canyon)
'criminal going around an unknown' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'going around' means one lot of letters goes inside another.
'unknown' becomes 'y' (algebra).
'an'+'y'='any'
'con' placed around 'any' is 'CANYON'.
(Other definitions for canyon that I've seen before include "grand spectacle in Arizona" , "gully" , "Ravine formed by a river" , "geographical feature" , "(N. Am.) ravine" .)