Quietly repeated advice to would-be captors (6)
I believe the answer is:
softly
'quietly' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for softly that I've seen before include "Quietly or gently" , "Lightly, quietly" , "In a quiet way" , "Piano" , "In a yielding manner" .)