Ace new doctor gone mad provides steroid (8)
I believe the answer is:
androgen
'steroid' is the definition.
(androgen is a kind of steroid)
'ace new doctor gone mad' is the wordplay.
'ace' becomes 'a'.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'doctor' becomes 'dr' (synonyms).
'mad' indicates anagramming the letters.
'gone' with letters rearranged gives 'ogen'.
'a'+'n'+'dr'+'ogen'='ANDROGEN'
'provides' acts as a link.
(Other definitions for androgen that I've seen before include "Male hormone" , "Steroid" .)