Chances of US mum fighting to keep husband largely the same (4,2,1,8)

'largely the same' is the definition.
(phrases comparing similar things)

'chances of us mum fighting to keep husband' is the wordplay.
'fighting' indicates anagramming the letters (I've seen this in other clues).
'to keep' indicates putting letters inside.
'husband' becomes 'h' (abbreviation).
'chances'+'of'+'us'+'mum'='chancesofusmum'
'chancesofusmum' with letters rearranged gives 'mucofamuchness'.
'mucofamuchness' going around 'h' is 'MUCH OF A MUCHNESS'.