Watch posted on risky boundaries (6)
I believe the answer is:
sentry
'watch' is the definition.
('sentry' can be a synonym of 'watch')
'posted on risky boundaries' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'on' is a charade indicator (letters next to each other).
'boundaries' says to hollow out the word (remove centre letters) (only the edge or boundary letters remain).
'risky' with its middle taken out is 'ry'.
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Watchperson" , "Soldier posted to keep watch" , "Look-out, guard" , "Person standing watch" , "Military guard" .)