Concern — appear plumb to umpire? (7)
I believe the answer is:
lookout
'concern' is the definition.
('that's your lookout')
'appear plumb to umpire?' is the wordplay.
'appear' becomes 'look' (synonyms).
'plumb to umpire?' becomes 'out' (plumb means definitely out in cricket).
'look'+'out'='LOOKOUT'
(Other definitions for lookout that I've seen before include "View" , "Person set to watch" , "Watchtower" , "Observer, sentinel" , "Observation point" .)