Bitterness one day encapsulated by a lament (8)
I believe the answer is:
acrimony
'bitterness' is the definition.
(I know that bitterness can be written as acrimony)
'one day encapsulated by a lament' is the wordplay.
'one' becomes 'i' (Roman numeral).
'day' becomes 'mon' (short for Monday).
'encapsulated by' is an insertion indicator (I've seen this in other clues).
'lament' becomes 'cry' (I've seen this in another clue).
'i'+'mon'='imon'
'a'+'cry'='acry'
'imon' inserted inside 'acry' is 'ACRIMONY'.
(Other definitions for acrimony that I've seen before include "Acronym I coined in hostility" , "Asperity" , "Bitter feeling" , "Bitterness of temper" , "Angry and unpleasant feelings or words" .)