Neither big nor small (6)
I believe the answer is:
medium
'neither big nor small' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for medium that I've seen before include "Moderate - agency - channel of communication" , "Average, around the middle of a scale" , "Spirits of dead people are said to communicate through one" , "Average kind of spiritualist" , "Intermediary between the living and the dead" .)