The old lady, impressed by military building's layout (6)
I believe the answer is:
format
'layout' is the definition.
(I've seen this in another clue)
'the old lady impressed by military building's' is the wordplay.
'the old lady' becomes 'ma' (both can informally mean mother).
'impressed by' indicates putting letters inside.
'military building' becomes 'fort' (I've seen this before).
'ma' placed inside 'fort' is 'FORMAT'.
(Other definitions for format that I've seen before include "Style or arrangement of procedure" , "Shape and size as of a book, lay-out" , "Shape and style as of book" , "Size, style and shape, of books say" , "Arrangement (data)" .)