Spirit used in an experiment (4)
I believe the answer is:
peri
'spirit' is the definition.
(I know that peri is a type of spirit)
'in an experiment' is the wordplay.
'in' indicates a hidden word.
'PERI' is hidden in the letters of 'experiment'.
'used' is the link.
(Other definitions for peri that I've seen before include "Prefix denoting 'around'" , "Old Persian fairy" , "Fairy could be ripe" , "Fairy queen" , "Malevolent Persian spirit" .)