End legal action to detain European (5)
I believe the answer is:
cease
'end' is the definition.
(I know that end can be written as cease)
'legal action to detain european' is the wordplay.
'legal action' becomes 'case' (a legal case).
'to detain' is an insertion indicator (inserted letters are held or detained).
'european' becomes 'E' (abbreviation e.g. EU).
'case' placed around 'e' is 'CEASE'.
(Other definitions for cease that I've seen before include "Culminate" , "Stop, finish" , "Desist" , "Finish, end" , "Stop, terminate" .)