Kept going first, in date order (10)
I believe the answer is:
'kept' is the definition.
('maintain' can be a synonym of 'keep')
'going first in date order' is the wordplay.
'going first' becomes 'main' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'order' indicates an anagram.
'indate' is an anagram of 'tained'.
Can you help me to learn more?
(Other definitions for maintained that I've seen before include "being kept going" , "Declared; supported" , "Insisted" , "Kept up (level)" , "Sustained" .)