Checked out of the library in the early 80s.
Hardback, tan cover with a cauldron on the front, green smoke coming out of the caldron and wrapping around the back.
An apprentice witch’s mistress is accused of some crime by other witches and taken away. The apprentice tries to find the real culprit. She is able to summon a frog-like helper who knows what is going on. Froggie at first cannot answer her questions, but hints broadly. She finds the next part of the spell and he is able to answer “yes” or “no”. Then she finds the last part and he can give full answers. (There is a wand or a book involved, I think)? If I’m not getting this confused with a different book, at some point she allies herself with two children from the circus who are trapeze artists.