Group of activists died in custody (5)
I believe the answer is:
cadre
'group of activists' is the definition.
(I've seen this before)
'died in custody' is the wordplay.
'died' becomes 'd'.
'in' indicates putting letters inside.
'custody' becomes 'care' (I've seen this before).
'd' put inside 'care' is 'CADRE'.
(Other definitions for cadre that I've seen before include "revolutionary cell" , "Raced about in military group" , "Raced around the framework of a military unit" , "Raced (anag) - small central group" , "Raced around with basic military unit" .)