Clue: Weird sisters pollarding birches, say (7)
Answer: WITCHES
Explanation: Definition: Weird sisters
SWITCHES (sticks used for beating, e.g. birch) "pollarded", i.e. with its top removed. The Weird Sisters are the three witches in *Macbeth*. (I spent a while trying to justify this as a homophone of "wyches", meaning certain types of tree)