Until then, against me first and last (8)
I believe the answer is:
meantime
'until then against' is the definition.
I can't tell whether this defines the answer.
'me first and last' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'and' becomes 'n' (common abbreviation for 'and').
'last' becomes 'time' (I can't justify this - if you can you should believe this answer much more).
'me'+'a'+'n'+'time'='MEANTIME'
Can you help me to learn more?
(Other definitions for meantime that I've seen before include "interval" , "interim?" , "For the moment" , "Intervening period" .)