Train seen going off the rails, seriously (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(synonyms)
'train seen going off the rails' is the wordplay.
'going off the rails' indicates anagramming the letters (the order of letters is out of control).
'train'+'seen'='trainseen'
'trainseen' with letters rearranged gives 'IN EARNEST'.
(Other definitions for in earnest that I've seen before include "resolutely" , "with enthusiasm" , "Sincere and serious" , "with serious intent" , "Not joking" .)