Watchful prince missing bishop (5)
I believe the answer is:
alert
'watchful' is the definition.
('alert' can be similar in meaning to 'watchful')
'prince missing bishop' is the wordplay.
'prince' becomes 'albert' (I've seen this before).
'missing' is a deletion indicator.
'bishop' becomes 'b' (abbreviation used in chess).
'albert' with 'b' taken away is 'ALERT'.
(Other definitions for alert that I've seen before include "Wide awake and watchful" , "Watchfully awake" , "Watchful, quick-thinking" , "Watchful, sharp" , "On one's toes" .)