Discovery means plea is changed (6)
I believe the answer is:
espial
'discovery' is the definition.
(espial is a kind of discovery)
'plea is changed' is the wordplay.
'changed' indicates anagramming the letters.
'plea'+'is'='pleais'
'pleais' anagrammed gives 'ESPIAL'.
'means' acts as a link.
(Other definitions for espial that I've seen before include "notice" , "secret observation" , "Seeing" , "Spotting" , "Detection" .)