No fewer than fifty invading a revolutionary state (2,5)
I believe the answer is:
at least
'no fewer than' is the definition.
(synonyms)
'fifty invading a revolutionary state' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'invading' means one lot of letters goes inside another.
'revolutionary' indicates anagramming the letters.
'state' is an anagram of '[state]'.
'a'+'[state]'='a[state]'
'l' inserted inside 'ateast' is 'AT LEAST'.
(Other definitions for at least that I've seen before include "No less than" , "Anyway" , "'If nothing else (2,5)'" , "however" , "Eat salt (anag) - if nothing else" .)