Serious art seen to become silly (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'art seen to become silly' is the wordplay.
'to become silly' indicates an anagram (a silly spelling).
'art'+'seen'='artseen'
'artseen' with letters rearranged gives 'EARNEST'.
(Other definitions for earnest that I've seen before include "Sincere; enthusiastic" , "Fixed" , "Seriously eager" , "Eager, sincere" , "Art worthy" .)