Supposedly the greatest length trunk road spans? (3)
I believe the answer is:
ali
'supposedly the greatest' is the definition.
(Muhammad Ali was known as the Greatest)
'length trunk road spans?' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'trunk road' becomes 'AI' (trunk road can mean a major road, eg the A1).
'spans?' indicates putting letters inside (span can mean to cross or go over).
'l' inserted inside 'ai' is 'ALI'.
(Other definitions for ali that I've seen before include "Muhammad ... (former fighter)" , ""The Greatest" boxer" , "girl shortly" , "Cassius Clay became Muhammad ..." , "Champion boxer" .)