Othello was said to be so secure in Venice (4)
I believe the answer is:
moor
'othello' is the definition.
(Shakespeare's Othello is a Moor)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for moor that I've seen before include "Heathery upland" , "Tether (a boat)" , "Old North African" , "Heathland" , "Fasten" .)