Fairy ran, briefly, backwards (3)
I believe the answer is:
elf
'fairy' is the definition.
(I know that elf is a type of fairy)
'ran briefly backwards' is the wordplay.
'ran' becomes 'fled' (to flee is to run away).
'briefly' means to remove the last letter.
'backwards' shows that the letters should be reversed in order.
'fled' with its final letter taken off is 'fle'.
'fle' backwards is 'ELF'.
(Other definitions for elf that I've seen before include "Imaginary small being with magic powers" , "German team?" , "Santa helper" , "Malignant fairy" , "A small sprite with magic powers" .)