Wife builds a tomb for marsupial (6)
I believe the answer is:
wombat
'for marsupial' is the definition.
(wombat is a kind of marsupial)
'wife builds a tomb' is the wordplay.
'wife' becomes 'w' (abbreviation).
'builds' indicates an anagram (I've seen 'build' mean this).
'a'+'tomb'='atomb'
'atomb' with letters rearranged gives 'ombat'.
'w'+'ombat'='WOMBAT'
(Other definitions for wombat that I've seen before include "Bear-like marsupial" , "Australian animal like small bear" , "Australian resident" , "Tailless marsupial down under" , "Australian animal resembling small bear" .)