Yobbo's stopped by good old watchman (7)
I believe the answer is:
lookout
'watchman' is the definition.
(I know that lookout is a type of watchman)
'yobbo's stopped by good old' is the wordplay.
'yobbo' becomes 'lout' (I've seen this before).
'stopped by' means one lot of letters goes inside another (stopping can mean blocking a gap).
'good' becomes 'ok'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ok'+'o'='oko'
'lout' going around 'oko' is 'LOOKOUT'.
(Other definitions for lookout that I've seen before include "Person on watch for dangers" , "Be careful!" , "Sentinel, watcher" , "Beware or gaze through window" , "A sentry or guard, must be watchful" .)