Pack’s overtaking East Asian runner (6)
I believe the answer is:
ganges
'asian runner' is the definition.
(River Ganges, something which 'runs')
'pack's overtaking east' is the wordplay.
'pack's' becomes 'gangs' ('gang' can be a synonym of 'pack').
'overtaking' means one lot of letters goes inside another (a word 'takes over' another set of letters).
'east' becomes 'e' ('E' can be a synonym of 'east').
'gangs' going around 'e' is 'GANGES'.
(Other definitions for ganges that I've seen before include "Holy river on India" , "Holy Indian river" , "Sacred river of N India" , "River flowing into the Bay of Bengal" , "Great river of India" .)