Drug otherwise used in eating regimens rejected (7)
I believe the answer is:
steroid
'drug' is the definition.
(I've seen this before)
'otherwise used in eating regimens rejected' is the wordplay.
'otherwise' becomes 'or'.
'used in' means one lot of letters goes inside another.
'eating regimens' becomes 'diets' (I can't justify this - if you can you should give a lot more credence to this answer).
'rejected' is a reversal indicator.
'or' put inside 'diets' is 'diorets'.
'diorets' backwards is 'STEROID'.
Can you help me to learn more?
(Other definitions for steroid that I've seen before include "might be abused in gym" , "Organic compound" , "Performance-enhancer illegal for athletes" , "Editors (anag.)" , "growth medication" .)