Cassowary's back to collect bones (4)
I believe the answer is:
ossa
'bones' is the definition.
(I've seen this before)
'cassowary's back to collect' is the wordplay.
'to collect' indicates a hidden word (I've seen 'collecting' mean this).
'back' says the letters should be written in reverse.
'asso' can be found hidden inside 'cassowary'.
'asso' written backwards gives 'OSSA'.
(Other definitions for ossa that I've seen before include "So as to become a Greek mountain" , "Greek mountains" , "Hard things" , "famous mount" , "To heap Pelion on -- is to compound difficulties" .)