Headdress that is at right angles initially (5)
I believe the answer is:
tiara
'headdress' is the definition.
(tiara is a kind of headdress)
'that is at right angles initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'that is at right angles' is 'TIARA'.
(Other definitions for tiara that I've seen before include "High diadem" , "Decorative headgear" , "Top decoration" , "expensive headwear" , "Jewelled, ornamental headgear" .)