A painter put on this oddly decorative headgear (5)
I believe the answer is:
tiara
'decorative headgear' is the definition.
(I've seen this before)
'a painter put on this oddly' is the wordplay.
'painter' becomes 'RA' (member of the Royal Academy of Arts).
'put on' is a charade indicator (letters next to each other) (some letters go on or after others).
'oddly' means one should take alternating letters (letters in odd-numbered positions).
The alternating letters of 'this' are 'ti'.
'a'+'ra'='ara'
'ara' after 'ti' is 'TIARA'.
(Other definitions for tiara that I've seen before include "special headgear" , "Ornamental headpiece" , "Half-crown?" , "Woman's head ornament" , "Half a crown!" .)