Apprehending a ringleader relaxing (9)
I believe the answer is:
arresting
'apprehending' is the definition.
('arrest' can be a synonym of 'apprehend')
'a ringleader relaxing' is the wordplay.
'ringleader' becomes 'r' (first letter of ring).
'relaxing' becomes 'resting' (I've seen this in another clue).
'a'+'r'+'resting'='ARRESTING'
(Other definitions for arresting that I've seen before include "Police officers are doing it regularly" , "Police work" , "Very striking" , "Copper's work" , "job of a police officer" .)