Rough cases fine on old watch (7)
I believe the answer is:
lookout
'watch' is the definition.
('lookout' can be a synonym of 'watch')
'rough cases fine on old' is the wordplay.
'rough' becomes 'lout' (both can mean a violent person**).
'cases' indicates putting letters inside.
'fine' becomes 'ok' ('OK' can be a synonym of 'fine').
'on' indicates putting letters inside (as in clothing \'on\' a person).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ok' enclosing 'o' is 'ook'.
'lout' enclosing 'ook' is 'LOOKOUT'.
(Other definitions for lookout that I've seen before include "Vantage point" , "One who keeps watch, a sentry" , "Outlook for sentry" , "Observer, sentinel" , "Sentinel, watcher" .)