Brief attack on retreating Yankee (6)
I believe the answer is:
notify
'brief' is the definition.
(thesaurus)
'attack on retreating yankee' is the wordplay.
'attack' becomes 'fit' (fit is a kind of attack**).
'retreating' says the letters should be written in reverse.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'fit'+'on'='fiton'
'fiton' reversed gives 'notif'.
'notif'+'y'='NOTIFY'
(Other definitions for notify that I've seen before include "Advise, apprise" , "No one" , "Formally tell" , "alert" , "Make known" .)