Bleak loch in storm — nothing escapes it (5,4)
I believe the answer is:
'nothing escapes it' is the definition.
(nothing can escape from a black hole)
'bleak loch in storm' is the wordplay.
'in storm' indicates anagramming the letters (I've seen this in other clues).
'bleakloch' with letters rearranged gives 'BLACK HOLE'.
(Other definitions for black hole that I've seen before include "Space left after collapse of a star" , "discovered in Calcutta" , "Astronomical curiosity" , "'Region of space from which nothing can escape (5,4)'" , "object in 21 [SPACE] having very strong 20 [GRAVITY]" .)