Princess climbing Congress wings (7)
I believe the answer is:
annexes
'wings' is the definition.
('annex' can be a synonym of 'wing')
'princess climbing congress' is the wordplay.
'princess' becomes 'Anne' (Princess Anne, daughter of Queen Elizabeth).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'congress' becomes 'sex' (I've seen this in another clue).
'sex' in reverse letter order is 'xes'.
'anne'+'xes'='ANNEXES'
(Other definitions for annexes that I've seen before include "Adds or joins to larger thing" , "outbuildings" , "Illegally seizes" .)