Cleopatra’s probably holding reptile (3)
I believe the answer is:
asp
'reptile' is the definition.
(asp is a kind of reptile)
'cleopatra's probably holding' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'ASP' can be found hidden inside 'cleopatras probably'.
(Other definitions for asp that I've seen before include "One secreting poison" , "Small poisonous snake" , "Slithery thing" , "creeper" , "Predatory fish" .)