Sarah's son in Pisa Academy (5)
I believe the answer is:
isaac
'sarah's son' is the definition.
(I've seen this in another clue)
'in pisa academy' is the wordplay.
'in' says the answer is hidden in the clue.
'ISAAC' is hidden in the letters of 'pisa academy'.
(Other definitions for isaac that I've seen before include "Put away" , "one almost sacrificed" , "Stern" , "According to the book, twins' dad" , "-- Asimov, science fiction writer" .)