Boundary marking the limits of a black hole (5,7)
I believe the answer is:
event horizon
'boundary marking the limits of a black hole' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for event horizon that I've seen before include "(In astronomy) notional boundary of a black hole - viz not hereon (anag)" , "(In astronomy) boundary of a black hole - no Heinz voter (anag)" , "Point of no return" , "black hole terminates here" .)