Regularly tell off mischievous child? (3)
I believe the answer is:
elf
'mischievous child?' is the definition.
(I've seen this in another clue)
'regularly tell off' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'tell'+'off'='telloff'
The alternate letters of 'telloff' are 'ELF'.
(Other definitions for elf that I've seen before include "Pixie (able to cobble?)" , "Fairy-like creature" , "'Pixie, imp (3)'" , "Mischievous youngster" , "Imaginary small being with magic powers" .)