Spirit used in experiments (4)
I believe the answer is:
peri
'spirit' is the definition.
(I know that peri is a type of spirit)
'in experiments' is the wordplay.
'in' indicates the answer is hidden within the clue.
'PERI' is hidden within 'experiments'.
'used' acts as a link.
(Other definitions for peri that I've seen before include "A vision of loveliness" , "Ripe as a kind of fairy" , "Fairy could be ripe" , "Old Persian fairy" , "Fairy of Persian folklore" .)