Signal judge harbouring Dutch convict (3,4)
I believe the answer is:
red flag
'signal' is the definition.
(thesaurus)
'judge harbouring dutch convict' is the wordplay.
'judge' becomes 'ref'.
'harbouring' indicates putting letters inside.
'dutch' becomes 'd' (common abbreviation).
'convict' becomes 'lag' (I've seen this before).
'ref' enclosing 'd' is 'redf'.
'redf'+'lag'='RED FLAG'
(Other definitions for red flag that I've seen before include "That should make you careful!" , "A flyer" , "Communist banner" , "Revolutionary symbol and song to the tune of Tannenbaum" , "Danger warning" .)