Ultimate of felons breaks marriage agreement (6)
I believe the answer is:
unison
'agreement' is the definition.
(I know that unison is a type of agreement)
'ultimate of felons breaks marriage' is the wordplay.
'ultimate of' suggests the final letters (I've seen 'ultimate' mean this).
'breaks' indicates putting letters inside (inserted letters break into the word).
'marriage' becomes 'union' (synonyms).
The last letter of 'felons' is 's'.
's' placed inside 'union' is 'UNISON'.
(Other definitions for unison that I've seen before include "In agreement - like sound of choir" , "Sameness in pitch" , "Accord" , "using same pitch" , "Trade union (where all agree?)" .)