Little marsupial? Gosh, it's big! (5)
I believe the answer is:
roomy
'big' is the definition.
(I've seen this before)
'little marsupial? gosh it's' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'marsupial?' becomes 'roo' (short for kangaroo).
'gosh' becomes 'my' (both are old-fashioned expressions of surprise).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'roo'+'my'+'e'='roomye'
'roomye' with its final letter removed is 'ROOMY'.
(Other definitions for roomy that I've seen before include "Spacious, can swing a cat in it" , "Without cramping" , "Comfortably large" , "Having big space" , "Commodious" .)