It meant a pound note (formerly, at the end of the war) (5)
I believe the answer is:
oncer
'it meant a pound note' is the definition.
The definition suggests a singular noun but the answer is not.
'formerly at the end of the war' is the wordplay.
'formerly' becomes 'once' (synonyms).
'at the end of' indicates one should take the final letters.
The last letter of 'war' is 'r'.
'once'+'r'='ONCER'
Can you help me to learn more?
(Other definitions for oncer that I've seen before include "Old note" , "Historic note" , "A sort of note" , "Australian MP (but not for long)" .)