Endlessly cross marsupial (3)
I believe the answer is:
roo
'marsupial' is the definition.
(I've seen this before)
'endlessly cross' is the wordplay.
'endlessly' means to remove the last letter.
'cross' becomes 'rood' (term for a crucifix).
'rood' with its last letter removed is 'ROO'.
(Other definitions for roo that I've seen before include "Australian jumper" , "Kanga's child (Pooh)" , "Marsupial (dim.)" , "Aussie jumper!" , "Small inhabitant of Hundred Acre Wood" .)