Serious alteration to sneer at (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'alteration to sneer at' is the wordplay.
'alteration to' indicates an anagram (the order of letters is altered).
'sneer'+'at'='sneerat'
'sneerat' with letters rearranged gives 'EARNEST'.
(Other definitions for earnest that I've seen before include "Serious in intention" , "Sneer at (anag) -- serious" , "Fixed" , "Sincere; enthusiastic" , "Sincere; determined" .)