Seen quite clearly at the Capitol, at least fifty were involved in a coup. (6)
I believe the answer is:
cupola
'seen quite clearly at the capitol at least' is the definition.
I can't judge whether this definition defines the answer.
'fifty were involved in a coup' is the wordplay.
I cannot quite understand how this works, but
'fifty' could be 'l' (Roman numeral) and 'l' is found within the answer.
'a' is present in the answer.
an anagram of 'coup' is 'cupo' which is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cupola that I've seen before include "Rounded dome" , "fireplace" , "Small dome on a roof" , "Domed ceiling" , "Round domed roof" .)