Nod so violently and it might fall off a girl's head (5)
I believe the answer is:
snood
'nod so violently and it might fall off a girl's head' is the definition.
(I've seen this before)
'nod so violently' is the wordplay.
'violently' indicates anagramming the letters.
'nod'+'so'='nodso'
'nodso' with letters rearranged gives 'SNOOD'.
(Other definitions for snood that I've seen before include "pouch" , "Nod so violently and it might fall off a girl's head" , "Hairnet; headband" , "Loose hairnet" , "Ornamental hairnet" .)