Old number from Hamlet hero (5)
I believe the answer is:
ether
'old number' is the definition.
(I've seen this in another clue)
'from hamlet hero' is the wordplay.
'from' indicates a hidden word.
'ETHER' is hidden in the letters of 'hamlet hero'.
(Other definitions for ether that I've seen before include "Chemical solvent" , "Light liquid used as anaesthetic" , "Anaesthetic; clear sky (poetic)" , "Some were put out by this" , "Imaginary interstellar medium" .)