Groups of agents in prison (5)
I believe the answer is:
cells
'groups of' is the definition.
'cells' can be an answer for 'groups' (I have seen 'terrorist groups' mean 'cells' so perhaps 'groups' could also mean 'cells'). I'm not sure about the 'of' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cells that I've seen before include "Small rooms" , "Accommodation for prisoners" , "Mobile phone in the US" , "Rooms for prisoners" , "terrorist groups" .)