… but not now, failing to exceed 26 down (2,6)
I believe the answer is:
no longer
'but not now' is the definition.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for no longer that I've seen before include "Not as formerly" , "One not disposed to pining" , "Someone who doesn't dream" , "a person without yearnings?" .)