Layout of a stronghold having a masterly occupant (6)
I believe the answer is:
format
'layout' is the definition.
(I've seen this in another clue)
'a stronghold having a masterly occupant' is the wordplay.
'a stronghold' becomes 'fort' (I've seen this in another clue).
'having' indicates putting letters inside.
'a masterly occupant' becomes 'ma' (I can't justify this - if you can you should give a lot more credence to this answer).
'fort' placed around 'ma' is 'FORMAT'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for format that I've seen before include "Arrangement or lay-out, say of book" , "Way in which something is presented" , "Style or arrangement of procedure" , "Broadsheet or tabloid, say" , "Design, lay-out" .)