It's deep inside wallaby's sack (5)
I believe the answer is:
'it's deep' is the definition.
'inside wallaby's sack' is the wordplay.
'inside' says the answer is hidden in the clue.
'ABYSS' is hidden within 'wallabys sack'.
(Other definitions for abyss that I've seen before include "Deep hole, seems bottomless", "great depth", "Bottomless gulf", "something rather deep", "Seemingly bottomless chasm".)