Watch nurse have a go (6)
I believe the answer is:
sentry
'watch' is the definition.
('sentry' can be a synonym of 'watch')
'nurse have a go' is the wordplay.
'nurse' becomes 'sen' (I've seen this before).
'have' is a charade indicator (letters next to each other).
'a go' becomes 'try' (I've seen this before).
'sen'+'try'='SENTRY'
(Other definitions for sentry that I've seen before include "Sentinel, watch" , "Sentinel, look-out" , "Lookout person" , "Look-out, guard" , "Military lookout" .)