Mysterious guy crossing channel after dreadful sin (11)
I believe the answer is:
inscrutable
'mysterious' is the definition.
(synonyms)
'guy crossing channel after dreadful sin' is the wordplay.
'guy' becomes 'cable' (I've seen this in other clues).
'crossing' is an insertion indicator.
'channel' becomes 'rut' (I've seen this before).
'after' means one lot of letters go next to another.
'dreadful' indicates an anagram.
'cable' enclosing 'rut' is 'crutable'.
'sin' with letters rearranged gives 'ins'.
'crutable' after 'ins' is 'INSCRUTABLE'.
(Other definitions for inscrutable that I've seen before include "difficult to understand" , "hard to understand" , "Mysterious" , "Impossible to understand" , "Difficult to get" .)