Ring head about one dope (6)
I believe the answer is:
opiate
'dope' is the definition.
(I've seen this before)
'ring head about one' is the wordplay.
'ring' becomes 'o'.
'head' becomes 'pate' (I've seen this before**).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pate' going around 'i' is 'piate'.
'o'+'piate'='OPIATE'
(Other definitions for opiate that I've seen before include "Drug inducing a false sense of contentment" , "A sedative drug" , "description of religion?" , "Narcotic drug" , "A poet I become on this drug" .)