Not clear about advertisement, English, for “Crown” (6)
I believe the answer is:
diadem
'for crown' is the definition.
(synonyms)
'not clear about advertisement english' is the wordplay.
'not clear' becomes 'dim'.
'about' indicates putting letters inside.
'advertisement' becomes 'ad' (short for advert).
'english' becomes 'e' (abbreviation).
'ad'+'e'='ade'
'dim' going around 'ade' is 'DIADEM'.
(Other definitions for diadem that I've seen before include "Ornamental jewelled headdress" , "Crown or headband worn as symbol of sovereignty" , "Royal headdress" , "Kingly crown" , "Crown or headband for sovereign" .)