Aleksandr, Sergei and Oleg are just pets, I hear (8)
I believe the answer is:
meerkats
'aleksandr sergei and oleg' is the definition.
(meerkat characters in CompareTheMarket adverts)
'are just pets i hear' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
'i' could be 'me' and 'me' is found in the answer.
This explanation may well be incorrect...
(Other definitions for meerkats that I've seen before include "Burrowing creatures" , "African mongooses" , "Strange tee marks made by mongoose-like animals" , "Strange tee marks made by the mammals" , "South African carnivores (nature's experts at 21 17!)" .)