Fox heard ET is grounded (7)
I believe the answer is:
earthed
'grounded' is the definition.
(electrical term)
'fox heard et' is the wordplay.
'fox' indicates anagramming the letters (a tricky or foxy version of the letters).
'heard'+'et'='heardet'
'heardet' is an anagram of 'EARTHED'.
'is' acts as a link.
(Other definitions for earthed that I've seen before include "grounded" , "attached to the ground" , "protected from shock" .)