Brief problem supported by Queen (7)
I believe the answer is:
summary
'brief' is the definition.
(I've seen this before)
'problem supported by queen' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'supported by' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'queen' becomes 'Mary' (Mary I or Mary II).
'sum'+'mary'='SUMMARY'
(Other definitions for summary that I've seen before include "Immediate" , "Precis, gist" , "Arbitrary" , "Resume" , "Synopsis, outline" .)