Signal to bring in a social worker (9)
I believe the answer is:
important
'signal' is the definition.
(thesaurus)
'bring in a social worker' is the wordplay.
'bring in' becomes 'import' (bring in to a country).
'a social worker' becomes 'ant' (ants are social insects).
'import'+'ant'='IMPORTANT'
'to' is the link.
(Other definitions for important that I've seen before include "Meaningful" , "It's vital" , "Noteworthy, significant" , "material" , "Noted" .)