Fails to keep a date but remains valid (6,2)
I believe the answer is:
stands up
'fails to keep a date' is the definition.
I can't judge whether this defines the answer.
'remains valid' is the wordplay.
'remains' becomes 'stands' (standing is a kind of remaining).
'valid' becomes 'up' (I can't justify this - if you can you should believe this answer much more).
'stands'+'up'='STANDS UP'
'but' is the link.
Can you help me to learn more?
(Other definitions for stands up that I've seen before include "Rises; bears examination" , "Ceases to sit, adopts vertical posture" , "fails to meet" , "Reacts to 15 [NATIONAL ANTHEM]" , "Rises; bears investigation" .)