Early gremlin, a failing (8)
I believe the answer is:
germinal
'early' is the definition.
'gremlin a failing' is the wordplay.
'failing' indicates an anagram.
'gremlin'+'a'='gremlina'
'gremlina' with letters rearranged gives 'GERMINAL'.
(Other definitions for germinal that I've seen before include "In the earliest stage of development, like new ideas" , "Novel by Zola; in an early stage of development" , "Relating to earliest stages of development" , "Embryonic; Zola novel" , "Rudimentary" .)