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 "Take without permission", "bargain?", "Nick", "Take unlawfully", "Plagiarise", "bag", "Filch", "Lift - pocket", "Pilfer" and "Make off with".)