Yankee after rejecting suitable working brief (6)
I believe the answer is:
notify
'brief' is the definition.
(thesaurus)
'yankee after rejecting suitable working' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'after' means one lot of letters go next to another.
'rejecting' is a reversal indicator.
'suitable' becomes 'fit' (similar in meaning).
'working' becomes 'on' (on can mean working or activated).
'fit'+'on'='fiton'
'fiton' reversed gives 'notif'.
'y' after 'notif' is 'NOTIFY'.
(Other definitions for notify that I've seen before include "Formally inform" , "alert" , "Officially inform" , "Declare" , "Formally tell" .)