No fewer than twelve ultimately entering after all (2,5)
I believe the answer is:
at least
'no fewer than' is the definition.
(synonyms)
'twelve ultimately entering after all' is the wordplay.
'ultimately' says to take the final letters.
'entering' is an insertion indicator.
'after all' becomes 'atlast' (both can mean eventually or finally).
The last letter of 'twelve' is 'e'.
'e' placed into 'atlast' is 'AT LEAST'.
(Other definitions for at least that I've seen before include "'If nothing else (2,5)'" , "Anyway" , "Eat salt (anag) - if nothing else" , "No less than" , "however" .)