Beirut is restored by an emperor (8)
I believe the answer is:
tiberius
'an emperor' is the definition.
(I know that Tiberius is a Roman emperor)
'beirut is restored' is the wordplay.
'restored' is an anagram indicator.
'beirut'+'is'='beirutis'
'beirutis' is an anagram of 'TIBERIUS'.
'by' is the link.
(Other definitions for tiberius that I've seen before include "Tyrannical successor to Emperor Augustus, d. AD 37" , "Roman emperor who succeeded Augustus" , "Emperor at time of Crucifixion" , "Augustus's successor" , "ruler" .)