Advised in fighting retreat having to be reversed (6)
I believe the answer is:
warned
'advised' is the definition.
(to advise someone can mean to warn them)
'fighting retreat having to be reversed' is the wordplay.
'fighting' becomes 'war'.
'retreat' becomes 'den' (both can mean a lair or hideout).
'having to be reversed' says the letters should be written backwards.
'den' backwards is 'ned'.
'war'+'ned'='WARNED'
'in' acts as a link.
(Other definitions for warned that I've seen before include "Cautioned about Andrew" , "Put on the alert" , "Notified of potential danger" , "Notified in advance" .)