Formerly a hundred in a single container (4)
I believe the answer is:
once
'formerly' is the definition.
(synonyms)
'a hundred in a single container' is the wordplay.
I cannot quite understand how this works, but
'hundred' could be 'c' (Roman numeral) and 'c' is found within the answer.
'a' could be 'one' and 'one' is located in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for once that I've seen before include "... upon a time for the storyteller" , "in bygone days" , "In days gone by" , "Formerly - not repeated" , "At some former time" .)