Poach starter of scallops and duck (5)
I believe the answer is:
Here is my best explanation:
'poach' is the definition.
'starter of scallops and duck' is the subsidiary indication.
'starter of' suggests taking the first letters.
'and' is a charade indicator (letters next to each other).
'duck' becomes 'teal' (teal is a kind of duck).
The first letter of 'scallops' is 's'.
(Other definitions for steal that I've seen before include "Snatch", "Take unlawfully", "Make off with", "Rob", "Take without permission", "Creep", "Thieve", "Take illegally", "Nick" and "Purloin".)