Is active in brewing! (4)
I believe the answer is:
hops
'is' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for hops that I've seen before include "what could make you bitter" , "Skips" , "Leaps on one leg" , "Dances -- beer ingredient" , "Shop around for beer material" .)