Speller's turn to forget first letter (5)
I believe the answer is:
witch
'speller's' is the definition.
(someone who casts spells)
'turn to forget first letter' is the wordplay.
'turn' becomes 'switch' (as in switching or turning something on).
'to forget first letter' means to remove the first letter.
'switch' with its initial letter taken off is 'WITCH'.
(Other definitions for witch that I've seen before include "Female member of coven" , "Enchantress" , "She has bad spells" , "Female sorcerer" , "Black cat and broomstick owner?" .)