Steal an advantage (4)
I believe the answer is:
edge
I believe this clue is a double definition.
'steal' is the first definition.
(I've seen this before)
'an advantage' is the second definition.
(having the edge is having an advantage)
(Other definitions for edge that I've seen before include "Border; intense quality" , "'Margin, rim (4)'" , "The part farthest from the middle" , "Border, fringe" , "Lip; advantage" .)