A mostly dreary time — but not for children (5)
I believe the answer is:
adult
'not for children' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for adult that I've seen before include "I wasn't born yesterday" , "not green" , "One no longer young" , "used to describe dirty movies" , "Full-grown" .)