Guard dog -- watch out! (6)
I believe the answer is:
secure
'guard' is the definition.
(thesaurus)
'dog watch out' is the wordplay.
'dog' becomes 'cur' (I've seen this before).
'watch' becomes 'see' (seeing is a kind of watching).
'out' is an insertion indicator.
'cur' put into 'see' is 'SECURE'.
(Other definitions for secure that I've seen before include "Safe, protected" , "Stable" , "Safe against attack or well fastened" , "properly restrained" , "Obtain - safe" .)