Rodent kept as a pet (6)
'rodent kept as a pet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hamster that I've seen before include "Rodent with large cheek pouches" , "Rodent often kept as pet" , "Little beast" , "Kind of rodent with cheekpouches, often a pet" , "one running round in circles?" .)
(Other definitions for gerbil that I've seen before include "Leg plus rib of a mouse-like desert rodent" , "Burrowing mouse-like rodent" , "Mouse-like rodent (G not J)" , "Small burrowing rodent" , "Burrowing desert rodent" .)