Scientist from Syracuse building bridges across island and sea (10)
I believe the answer is:
archimedes
'scientist from syracuse' is the definition.
(Archimedes was born in Syracuse)
'bridges across island and sea' is the wordplay.
'bridges' becomes 'arches'.
'across' is an insertion indicator.
'island' becomes 'i' (geographical abbreviation).
'and' says to put letters next to each other.
'sea' becomes 'med' (short for Mediterranean).
'i'+'med'='imed'
'arches' placed around 'imed' is 'ARCHIMEDES'.
'building' acts as a link.
(Other definitions for archimedes that I've seen before include "Ancient Greek scientist (of buoyancy principle)" , "Man of principle" , "Old scientist" , "Ancient Greek mathematician/inventor" , "Syracusan" .)