Furtiveness shown by duck in south (7)
I believe the answer is:
stealth
'furtiveness' is the definition.
(I've seen this in another clue)
'duck in south' is the wordplay.
'duck' becomes 'teal' (teal is a type of duck).
'in' indicates putting letters inside.
'south' becomes 'sth' (abbreviation for south).
'teal' placed within 'sth' is 'STEALTH'.
'shown by' is the link.
(Other definitions for stealth that I've seen before include "Quiet or secret way of behaviour" , "Cautious and surreptitious action or movement" , "Avoidance of detection by moving carefully" , "quality needed by cat burglars" , "Kind of indirect tax" .)