In a frenzy: pet adder's gone (8)
I believe the answer is:
departed
'gone' is the definition.
('departed' can be a synonym of 'gone')
'in a frenzy pet adder's' is the wordplay.
'in a frenzy' indicates an anagram.
'pet'+'adder'='petadder'
'petadder' is an anagram of 'DEPARTED'.
(Other definitions for departed that I've seen before include "Left -- late" , "Left, went away" , "Gone, maybe dead" , "completely breathless" , "Went - dead" .)