Separatists ate out while under arrest (8)
I believe the answer is:
detained
'while under arrest' is the definition.
'separatists ate out' is the wordplay.
'separatists' becomes 'eta' (ETA, Basque separatist group).
'ate' becomes 'dined' (dining is a kind of eating).
'out' means one lot of letters goes inside another.
'eta' placed within 'dined' is 'DETAINED'.
(Other definitions for detained that I've seen before include "Kept (in custody)" , "Deaden it (anag.)" , "Under restraint" , "Held in Custody" , "Remanded or delayed" .)