Fail to keep up a legend (4)
I believe the answer is:
saga
'legend' is the definition.
(I've seen this in another clue)
'fail to keep up a' is the wordplay.
'fail to keep' becomes 'gas' (I can't justify this - if you can you should give a lot more credence to this answer).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'gas' backwards is 'sag'.
'sag'+'a'='SAGA'
Can you help me to learn more?
(Other definitions for saga that I've seen before include "Epic tale" , "Tale of heroes" , "Epic story" , "Epic - legend" , "long series of events" .)