Indicator of Falangists retreating (6)
I believe the answer is:
signal
'indicator' is the definition.
(thesaurus)
'of falangists retreating' is the wordplay.
'of' says the answer is hidden in the clue.
'retreating' shows that the letters should be reversed in order.
'langis' is hidden within 'falangists'.
'langis' reversed gives 'SIGNAL'.
(Other definitions for signal that I've seen before include "Non-verbal action with an encoded message" , "important to train drivers > striking" , "Noteworthy" , "Sign that conveys a message" , "Notable; warning sign" .)