Something often repeated quietly — a hidden danger? (7)
I believe the answer is:
mantrap
'a hidden danger?' is the definition.
(trap for catching people)
'something often repeated quietly' is the wordplay.
'something often repeated' becomes 'mantra' (a mantra is a repeated phrase or incantation).
'quietly' becomes 'p' (p for piano and quiet).
'mantra'+'p'='MANTRAP'
(Other definitions for mantrap that I've seen before include "dish?" , "Catch people using this" , "poacher may have been held" , "Illegal snare" , "People may be caught by this" .)