Popular sport's first ruling: sides banned for hormone (7)
I believe the answer is:
insulin
'hormone' is the definition.
(I know that insulin is an peptide hormone)
'popular sport's first ruling sides banned' is the wordplay.
I cannot quite understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for insulin that I've seen before include "Hormone or medication controlling body's sugar level" , "Hormone - nil in us (anag)" , "Treatment for diabetes" , "Hormone that controls the body's sugar absorption" , "Diabetes drug" .)