Serious, sincere (7)
I believe the answer is:
I believe this clue is a double definition.
'serious' is the first definition.
(similar in meaning)
'sincere' is the second definition.
(I know that sincere can be written as earnest)
(Other definitions for earnest that I've seen before include "Sincere; enthusiastic" , "Intensely serious; token" , "Art worthy" , "money involved in contract" , "Showing sincere and intense conviction" .)