Kitten group in a sort of pile (6)
I believe the answer is:
atomic
This clue is a double definition.
'kitten group' is the first definition.
(Atomic Kitten are a pop group)
'a sort of pile' is the second definition.
(nuclear physics)
'in' acts as a link.
(Other definitions for atomic that I've seen before include "Basically simple" , "Realting to nuclear energy" , "Employing nuclear energy" , "Sort of bomb, number, or power" , "Relating to tiny particles" .)