Element detected around large island with uniform mass (8)
I believe the answer is:
selenium
'element' is the definition.
(selenium is a kind of element)
'detected around large island with uniform mass' is the wordplay.
'detected' becomes 'seen'.
'around' indicates putting letters inside.
'large' becomes 'l'.
'island' becomes 'i'.
'with uniform mass' becomes 'um' (I am not sure about this - if you are sure you should believe this answer much more).
'seen' going around 'l' is 'selen'.
'selen'+'i'+'um'='SELENIUM'
(Another definition for selenium that I've seen is " Semiconducting element, symbol Se".)