Leaders from Rome attended debate in Ischia; more than one spoke (5)
I believe the answer is:
radii
'more than one spoke' is the definition.
(I've seen this before)
'leaders from rome attended debate in ischia' is the wordplay.
'leaders from' suggests taking the first letters.
The initial letters of 'rome attended debate in ischia' is 'RADII'.
(Other definitions for radii that I've seen before include "lines from a central part" , "spoke with others" , "Lines from centre to perimeter of a circle" , "Half diameters" , "Halves of diameter" .)