Appearance of mother in castle (6)
I believe the answer is:
format
'appearance' is the definition.
(I know that format is a type of appearance)
'mother in castle' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'in' means one lot of letters goes inside another.
'castle' becomes 'fort' (fort is a type of castle).
'ma' placed inside 'fort' is 'FORMAT'.
'of' acts as a link.
(Other definitions for format that I've seen before include "Broadsheet or tabloid, say" , "The way something is arranged or set out" , "The shape and size, of a book say" , "Secure firmly in position" , "Way of presentation" .)