Kid in group climbing races (6)
I believe the answer is:
tribes
'races' is the definition.
(I've seen this before)
'kid in group climbing' is the wordplay.
'kid' becomes 'rib' (I've seen this before).
'in' indicates putting letters inside.
'group' becomes 'set' (both can mean a collection).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'set' in reverse letter order is 'tes'.
'rib' placed inside 'tes' is 'TRIBES'.
(Other definitions for tribes that I've seen before include "Groups of families in traditional societies" , "Groups of people related by blood" , "Communities of primitive or nomadic groups" , "races" , "Racial groups living in communities headed by chiefs" .)