Frightful girl is silly from time to time (6)
I believe the answer is:
grisly
'frightful' is the definition.
(I've seen this before)
'girl is silly from time to time' is the wordplay.
'from time to time' indicates alternate letters (letters removed on a regular basis).
'girl'+'is'+'silly'='girlissilly'
The alternating letters of 'girlissilly' are 'GRISLY'.
(Other definitions for grisly that I've seen before include "Horrible, ghastly" , "'Gruesome, macabre (6)'" , "Frightful, ghastly" , "Causing horror or revulsion" , "Horrible, gruesome" .)