Sneer at failing firm (7)
I believe the answer is:
earnest
'firm' is the definition.
(I've seen this before)
'sneer at failing' is the wordplay.
'failing' is an anagram indicator.
'sneer'+'at'='sneerat'
'sneerat' is an anagram of 'EARNEST'.
(Other definitions for earnest that I've seen before include "Determined - heartfelt" , "Serious in intention" , "Eastern (anag.)" , "showing seriousness" , "promise" .)