Rock is a 50-carat amalgam, middle-sized (8)
I believe the answer is:
alcatraz
'rock' is the definition.
(Alcatraz prison was nicknamed the Rock)
'a 50-carat amalgam middle-sized' is the wordplay.
'50' becomes 'l' (Roman numeral).
'amalgam' indicates anagramming the letters.
'middle' says to take the centre.
The central letter of 'sized' is 'z'.
'carat' is an anagram of 'catra'.
'a'+'l'+'catra'+'z'='ALCATRAZ'
'is' acts as a link.
(Other definitions for alcatraz that I've seen before include "(Former prison) island in San Francisco Bay" , "US island, formerly a famous prison" , "Birdman's prison" , "Once-notorious prison in San Francisco Bay" , "Pen" .)