It’s most unlikely one’s to adopt Broad Church article (1,3,6)

'it's most unlikely' is the definition.
I know nothing about this answer so I can't judge whether this works.

'one's to adopt broad church article' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'to adopt' indicates putting letters inside (inserted letters are taken in or adopted).
'broad' becomes 'fat' (I can't explain this - if you can you should give a lot more credence to this answer).
'church' becomes 'ch' (common abbreviation).
'article' becomes 'an' (indefinite article).
'fat'+'ch'+'an'='fatchan'
'ace' enclosing 'fatchan' is 'A FAT CHANCE'.

Can you help me to learn more?