Day in custody cell (5)
I believe the answer is:
cadre
'cell' is the definition.
('cadre' can be a synonym of 'cell')
'day in custody' is the wordplay.
'day' becomes 'd'.
'in' means one lot of letters goes inside another.
'custody' becomes 'care' (I've seen this before).
'd' inserted into 'care' is 'CADRE'.
(Other definitions for cadre that I've seen before include "Raced around with basic military unit" , "Raced about in military group" , "Nucleus, framework" , "Fighting unit" , "Group of revolutionaries" .)