Pisa can't be rebuilt by leaders (8)
I believe the answer is:
captains
'leaders' is the definition.
(captain is a kind of leader)
'pisa can't be rebuilt' is the wordplay.
'be rebuilt' indicates anagramming the letters.
'pisa'+'cant'='pisacant'
'pisacant' with letters rearranged gives 'CAPTAINS'.
'by' acts as a link.
(Other definitions for captains that I've seen before include "''The tumult and the shouting dies / The ... and the Kings depart'' (Kipling)" , "Officers below major" , "Leads (a team)" , "Commanders of troops" , "Officers, in command of ships say (8)" .)