Watch bear joining sentry (5,5)
I believe the answer is:
stand guard
'watch' is the definition.
(I know that stand guard has the is a more specific form of the action guard)
'bear joining sentry' is the wordplay.
'bear' becomes 'stand' (synonyms).
'joining' says to put letters next to each other.
'sentry' becomes 'guard' (I've seen this before).
'stand'+'guard'='STAND GUARD'
(Other definitions for stand guard that I've seen before include "Keep watch" , "Act as sentry" , "Police" .)