Mater's off with mother's capital! (9)
I believe the answer is:
amsterdam
'capital' is the definition.
(Amsterdam is an example)
'mater's off with mother's' is the wordplay.
'off' indicates anagramming the letters.
'with' says to put letters next to each other.
'mother' becomes 'dam' (female parent of an animal).
'maters' anagrammed gives 'amster'.
'amster'+'dam'='AMSTERDAM'
(Other definitions for amsterdam that I've seen before include "Netherlands capital" , "place of canals" , "City of the Rijks-museum and Van Gogh Museum" , "diamond-cutting centre" , "city with many canals" .)