It's deep inside wallaby's sack (5)
I believe the answer is:
abyss
'it's deep' is the definition.
(thesaurus)
'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 "depths of sea" , "something rather deep" , "Bottomless gulf" , "Deep hole, seems bottomless" , "Bottomless pit" .)