Drug present in diets or otherwise (7)
I believe the answer is:
steroid
'drug present' is the definition.
'steroid' can be an answer for 'drug' (type of drug). I'm not sure about the 'present' bit.
'diets or otherwise' is the wordplay.
'otherwise' indicates an anagram.
'diets'+'or'='dietsor'
'dietsor' anagrammed gives 'STEROID'.
'in' is the link.
Can you help me to learn more?
(Other definitions for steroid that I've seen before include "might be abused in gym" , "Type of drug, maybe anabolic" , "muscle-building compound" , "Controversial body-builder" , "Type of hormone" .)