Seriously converted nine rates (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(synonyms)
'converted nine rates' is the wordplay.
'converted' indicates an anagram.
'nine'+'rates'='ninerates'
'ninerates' is an anagram of 'IN EARNEST'.
(Other definitions for in earnest that I've seen before include "resolutely" , "with enthusiasm" , "showing determination" , "With serious intention, not fooling about (2.7)" , "Not joking" .)